OIL-FINDING
OIL-FINDING
AN INTRODUCTION TO THE GEOLOGICAL
STUDY OF PETROLEUM
BY
E. H. CUNNINGHAM CRAIG, B.A,, F.G.S.
LATE OF H.M. GEOLOGICAL SURVEY
WITH AN INTRODUCTION
BY
SIR BOVERTON REDWOOD, BART.
ADVISER ON PETROLEUM TO THE ADMIRALTY, HOME OFFICE, AND INDIA OFFICE
CONSULTING ADVISER TO THE COLONIAL OFFICE
ILLUSTRATE?
LONDON
EDWARD ARNOLD
1912
{All rights reserved}
TO
AKTHUR SANKEY KEID
IN GRATEFUL MEMORY
OF HOW MUCH I OWE TO HIM
CONTENTS
PAGE
INTRODUCTION BY SIR BOVERTON REDWOOD, BART. . . vii
PREFACE ix
CHAPTER
I. THE ORIGIN OF PETROLEUM . . „ . . ~ . . l
H. PROCESSES OF FORMATION . 25
III. THE MIGRATION, FILTRATION, AND SUBTERRANEAN STORAGE
OF PETROLEUM .' . 38
IV. LATERAL VARIATION . . . . . . ... 56
V. GEOLOGICAL STRUCTURE 67
VI. INDICATIONS OF PETROLEUM ....... 88
VII. STRATIGRAPHY ." 121
VIII. LOCATION OF WELLS . 132
IX. (FOR BEGINNERS) FIELDWORK . . ... . 151
X. (FOR BEGINNERS) INDOOR-WORK ...... 176
INDEX ..... 191
254549
INTRODUCTION
BY
SIR BOVERTON REDWOOD, BART.
MR. CUNNINGHAM CRAIG'S "Oil-finding" is a laudable and
successful attempt to deal with a subject which has hitherto
received far too little attention, and those who have within
recent years had occasion to deplore the waste of money which
has resulted from the publication of injudicious reports on
lands presumed to be oil-bearing, and from the unscientific
manner in which drilling operations have been conducted, will
regret that this book was not published long ago.
The author points out that the foundation of the successful
petroleum enterprise must be laid by the geologist rather
than by the engineer, and he states that the present work has
been written for geologists, and especially for young geologists ;
but it may be added that, to a large extent, the views expressed
are couched in language so free from technicalities that the
work may be studied with profit by a far wider circle, including,
in fact, all those who are interested in the petroleum industry,
either in an administrative capacity, or as investors.
With the aid of this book, and the exercise of common
sense, those who contemplate investment in petroleum under-
takings may place themselves in a position to form an in-
dependent opinion as to whether the technical data given in
a prospectus are adequate, and are such as to justify the appeal
for subscriptions. Similarly, the shareholders in petroleum
projects which are of an exploratory nature, or in which the
work of exploitation is passing through the earlier stages, may
learn to interpret reports of progress which at present they
find unintelligible. Not only would this enlightened judgment
be of inestimable value to those who exercise it, but it would
incidentally provide the most effective remedy for an evil to
viii INTRODUCTION
which the author alludes, viz. that of the <; popular safeguarded
report/' which in lieu of being a record of facts and legitimate
deductions, seeks to create a highly favourable impression by
means of a few well-rounded sentences, in which the weak
points of the case are ignored and superlatives are prominent.
It is obvious that the possession by those to whom such reports
are intended to appeal of such knowledge as Mr. Cunningham
Craig's book imparts, would very soon result in rendering this
procedure worse than useless, and in this connexion it should
be borne in mind that the expert is not alone to blame, for,
as the author points out, it not infrequently happens that
his views are quoted in a form in which they do not correctly
convey what he stated.
The earlier portion of the work, as the author himself
admits, deals with many theoretical questions of a controversial
character, and it is not to be expected, nor does he himself
anticipate, that his strongly expressed opinions will in all cases
be accepted ; but this does not detract from the value of the
work, and in fact it may be said to enhance it if, in accordance
with the avowed intention of the author, the further study
of these questions is thereby stimulated.
Mr. Cunningham Craig in his concluding remarks modestly
disclaims originality for the last two chapters of this work,
and further indicates elsewhere that these chapters are in-
tended for beginners, but the dominant feature of the whole
work may be said to be the boldness and originality of
treatment of the subject, and there is much in the advice
which he gives to beginners which may be studied with
advantage by those who have had lengthy experience,
B. E.
AUTHOR'S PREFACE
THIS little book does not pretend to be a treatise upon
Petroleum, nor even to exhaust the particular circumscribed
branch of the subject with which it deals.
Petroleum and the search for petroleum have recently
bulked largely in the public eye. Discoveries all over the
world have proved the wide-spread occurrence of mineral oils ;
the demand for them has stimulated, and to some extent
created, active search for and development of petroliferous
areas, and it has become increasingly evident, not only to the
scientific, but also to the commercial world, that it is to the
geologist rather than to the engineer that one must look in
the first instance if successful results are to be achieved.
Much has been written in late years upon the subject of
petroleum, but very little that is of service to the practical
field-geologist. Speculative and more or less theoretical work
by indoor students of the subject is available in abundance,
while the practical work of oilfields has been in most cases con-
ducted by what have been known by a curious distinction (too
frequently heard even now-a-days), as " practical " as contrasted
with, or even opposed to, " scientific " men ; consequently many
facts of vital importance, and many generalizations reached by
long experience never see the light in publications accessible
to the scientific student of the subject. It is hardly too much
to say that so much nonsense has been written and published
about oil, or particular oilfields, that many vague but essentially
erroneous ideas are current, if not actually accepted. In many
otherwise excellent works on the subject geology is dismissed
in a few carefully guarded and colourless paragraphs, or at
the best in a chapter or two, while such data as statistics are
given a prominence which is more acceptable to the commercial
than to the scientific world. Similarly, geographical data are
set forth at length, and geological maps are rarities.
x AUTHOR'S PREFACE
It is to fill up a few of the blanks left by previous authors
that this book has been written ; it is for geologists, and more
especially for young geologists, in the hope that it may prove
of value to those who may have to undertake exploration work
for a mineral which is not naturally familiar to the stay-at-home
Briton.
The author is quite aware that his knowledge of petro-
leum is limited, but in the few not unimportant fields that
have come within his ken a thorough and intimate know-
ledge has been obtained, and he has been able to study the
subject in these fields in a manner for which neither the
--'T ^?/>^'< :.,• v^
ii. MUD-VOLCANO IN ERUPTION, TRINIDAD.
THE ORIGIN OF PETROLEUM 19
and covered by argillaceous beds. This dip brings the out-
crop below tide mark where the strata become horizontal. At
a distance of less than one hundred yards the strata emerge
with a low dip in the opposite direction, thus forming a very
gentle local syncline. Where the strata emerge the argillaceous
beds have thinned out or become replaced by arenaceous strata,
and the beds beneath are no longer burnt but consist of
FIG. 1. — Coast section west of Irois' (Trinidad). (Length about 200 yds.)
1. Porcellanite ; 2. Clay; 3. Impure lignite and shale ; 4. Sandstone.
carbonaceous shales with one seam of impure lignite. This
section can be observed from the local gulf steamer on its daily
route from San Fernando to Cedros and back, and can be studied
in detail during a walk along the beach. The whole section is
some two hundred yards in length, and is so well exposed that
there is no possibility of misunderstanding.
Such evidence places beyond doubt the connection between
the lignitic and petroliferous phases of these Tertiary strata,
and emphasises once again the point that a slight difference in
environment, the change from an arenaceous, that is to say, a
porous, cover to an argillaceous or impervious cover, seems to
determine whether the strata have ignited and burnt to porcel-
lanites or have remained as unburnt lignitic shales. It is
obvious that where strata lie at low angles the presence of an
impervious cover will tend to preserve any combustible or
volatile matter that may be in evidence in the underlying
strata from being rapidly dissipated or removed by weathering,
and thus will favour a slow combustion if a temperature suffi-
cient to cause ignition be reached.
We may safely conclude, then, that these " porcellanites "
of Trinidad represent a transition stage between the purely
petroliferous and the purely carbonaceous phases, they have
been more or less bituminous shales, and to attribute their
combustible matter to an animal origin would be the most
unjustifiable of assumptions.
The above evidence, selected from a mass of similar details.
20 OIL-FINDING
is sufficient to prove that in known oilfields the equivalence of
lignitic and petroliferous beds under slightly varying conditions
is indisputable. It remains now to show that in known coal-
fields, the association of petroleum with carbonaceous strata is,
though perhaps rare, by no means unprecedented. The point
to be considered is the environment, the conditions to which the
vegetable matter has been subjected. There are very many
instances on record of a series being petroliferous in the lower
beds, and lignitic or coal-bearing in the upper members. In
such cases it will always be found that a greater or less
thickness of comparatively impervious strata intervenes between
the two phases. The section at Point Ligoure on the Western
coast of Trinidad shows this very clearly, while in Borneo,
Eussia, West Virginia, and many other countries, lignite or
coal characterizes the less loaded or less perfectly sealed
horizons of a series. In the latter country oil wells are some-
times drilled through workable coal-seams, and the bores have
to be cased carefully to prevent water entering the coal-seams
and flooding the coal workings.
The evidence as to environment is confirmed by recent
researches on the nature of coals, and the conditions under
which they are found. It was until recently the accepted view
that anthracites are characteristic of the most disturbed,
contorted, and faulted parts of a coalfield, and that bituminous
coals are characteristic of the less disturbed portions. This
theory, though there seemed at one time to be ample evidence
for it, can no longer be held owing to the careful researches of
H.M. Geological Survey in the South Wales and Staffordshire
coalfields. Anthracites occur in comparatively slightly disturbed
strata in South Wales, and bituminous coal at the same horizons
in less disturbed areas. It has been suggested that differences
in the original conditions of deposition may account for this,
particularly as the amount of ash — inorganic material included
in the coal — varies at the same time, being greater in the
bituminous coals. Probably a truer explanation is to be sought
for in the fact that the anthracites occur where the coal has
been open to the influence of deep-seated weathering, and
where the structure and nature of the covering have favoured
the loss of volatile constituents. The greater original purity
of the deposit is also a factor to be reckoned with ; a pure coal
will readily give up its volatile or bituminous contents, while
THE ORIGIN OF PETROLEUM 21
an impure coal, owing to the " adsorptive " capacity of finely
divided inorganic matter for bitumen, retains it to a much
greater extent and will not part with it altogether, even under
the action of organic solvents.
Thus it is in deep mines where the seams do not crop out
at the surface, but are well sealed up beneath impervious
strata, no matter how contorted, that we must look for evidence
of petroleum. And the evidence, though somewhat scanty at
present, and unfortunately not always recorded, is not wanting.
Miners who have worked in deep workings of bituminous coal
tell of " tarry oozings " from the neighbourhood of seams, or
along joints in hard close-grained strata. Quite recently a
seepage of petroleum has been recorded from the Sovereign Pit
of the Wigan Coal and Iron Co., at Leigh, Lancashire, at a
depth of some 600 yards.
The oil-shales of Scotland, though not oilrocks in the strict
sense, add their quota to the mass of evidence connecting
petroleum and coal. These shales are contemporaneous with
coal-bearing strata, and though they have never, perhaps, been
actually petroliferous, they by their nature have been enabled
to retain in great quantity the material which under different
conditions would have been vast stores of liquid hydrocarbons.
The association of coal seams and oil-shale beds is so frequent,
e.g. in the Wolgan Valley in Australia, that it is unnecessary
to enlarge upon this point.
Thus we see that though coal and lignite are very different
substances from liquid petroleum, they are inextricably con-
nected ; coalfields give evidence of oil and oilfields of coal,
transitional stages can be searched for and found, and both
asphaltic and paraffin oils are seen impregnating the same
strata which at no great distance are carbonaceous in character.
Such evidence, almost always forthcoming as the result of
careful and detailed stratigraphical work in any part of the
world where petroleum is to be found, makes it hardly
possible to doubt that it is to terrestrial vegetation that we
must look for the raw material from which our supplies of
petroleum are derived.
But while stating this conclusion it must be borne in mind
that in certain cases, as for instance gas coals and oil- shales,
it is quite probable that such animal matter as may have been
preserved has borne its, very minor, part. The ammonia
22 OIL-FINDING
derivable from a gas coal or some of the oil-shales would
certainly suggest that some animal matter may have been
present. But all oil-shales do not contain ammonia in com-
bination, and petroleum is almost entirely free from nitrogenous
compounds, so that we may regard the ammonia contents
rather as adventitious than as essential. The Kimeridge clay
would certainly be mined as an oil-shale and distilled, were
it not for the absence of ammonia, which, fixed as the sulphate,
is the most valuable by-product of the Scottish oil-shales.
B. (2) Vegetable Origin. — Before leaving this branch of the
subject, it is necessary to refer to an idea or hypothesis
frequently put forward in a rather indefinite manner, but which
has found favour with many, especially those who have little
field experience.
This hypothesis is that petroleum is formed from marine
vegetation ; in other words seaweeds or fucoids. It was
apparently the desire to find some marine origin for oil that
caused this theory to be taken up, and allusions to " fucoids "
by writers on the subject of petroleum were at one time very
frequent.
But the origin of the theory was a series of observations
made on decomposing seaweed on the coasts of Sicily, Sardinia,
Norway, and other countries, where a "jelly-like substance"
was found to be formed at one stage of the subaerial decom-
position. This "jelly-like " matter was somewhat loosely
described as " substances resembling petroleum," and the
theory of a seaweed origin for oil sprang to birth in the minds,
not of the observers themselves, but of others who read about
it. The theory was again revived by the discovery of " Nhangel-
lite " formed in Portuguese South Africa, by the decomposition
of fresh water algaa in dried-up shallow lakes, and the claim
that this Nhangellite was evidence of the existence of petroleum
in the neighbourhood. In 1906 the author was asked to
investigate this claim in the field, but the evidence seemed
insufficient to justify the necessary expenditure of time.
So far as geological field evidence has been adduced in
favour of this theory, it has been confined to the production
of a few specimens of so-called fucoids, but in most cases, as
in the famous case of the Cambrian Fucoid Beds of the north-
west of Scotland, examination has proved the so-called "fucoids "
to be worm tracks and burrows. The exposure of this evidence,
THE ORIGIN OF PETROLEUM 23
however, has not entirely removed the theory from currency :
a theory can, it appears, survive the loss of the last piece of
direct evidence in its favour.
Let us again appeal to the facts and consider what evidence
can be brought up for and against a seaweed origin for petroleum.
In the first place, are there any inherent improbabilities in the
theory ? Is it possible for seaweed to be accumulated in vast
quantities and entombed in sediments as they are deposited ?
That vast quantities would be required will be admitted, as by
far the greater part by weight of seaweed, about seven-eighths,
is water.
Under what conditions do seaweeds flourish most luxuri-
antly ? It is a simple matter of observation. On rocky coasts,
in comparatively clear water, and in stagnant marine areas
such as the Sargasso Sea, seaweed can grow abundantly. But
in neither case is there any probability of the seaweed sinking
and becoming entombed in sediment. On rocky coasts the
weed is torn off by storms and cast on the shore, e.g. in the
west of Scotland and Ireland, where kelp-gathering is a regular
industry. In the Sargasso Sea the weed is floating or attached
to floating timbers, the remains of derelicts, etc.
In deep water, beyond the laminarian zone, seaweeds a*e
rare, small, and insignificant. In muddy estuaries, under
deltaic conditions, which have been proved to be the environ-
ment in which strata now oil-bearing have accumulated, where
in fact sedimentation proceeds, apace, there would seem to be
some possibility of weeds becoming involved and preserved in
the rapidly forming deposits, but in such conditions the waters
are singularly free from seaweed growth. Thus the initial
difficulty of postulating the possibility of a sufficient quantity
of raw material is, perhaps, even greater in the case of the
marine- vegetation theory than in the case of the animal- origin
theory.
Turning to chemical evidence, there are facts even more
difficult to explain away. When the water is removed from
seaweed, of the remaining solids a considerable proportion is
bromine and iodine in the form of salts. In fact, it is from the
ash of seaweeds that these elements are extracted commercially.
If petroleum is formed from the remains of seaweed, what
becomes of these bromides and iodides which must be present
in enormous quantity ? In one case a trace of iodine has been
24 OIL-FINDING
detected in the water from a mud- volcano, but the proportion
was quite insignificant compared with the trace of petroleum in
the same water.
The marine-vegetation theorists must account for the loss or
disappearance of these salts before they can justify the chemical
possibility of their hypothesis. Again, practically every
sample of petroleum that has ever been analysed contains some
trace of sulphur, and the percentage rises to three or more in
some cases. But there is no sulphur in seaweeds.
The chemical difficulties to be surmounted are therefore as
insurmountable as the initial difficulty of accumulation in
sufficient quantity.
If field evidence of unimpeachable character were available,
the matter would be worthy of serious consideration ; if fucoids
and traces of fucoids were found in quantity throughout a
series, and only disappeared among strata actually petroliferous,
it would be necessary to give special attention to the role
played by that class of organism and the strata in which the
evidence occurs, but when most, if not all, of the so-called
"fucoids" are worm-casts and tracks of animal organisms, the
practical geologist is unable to treat the theory with respect.
Thus every hypothesis but that of the origin from terrestrial
vegetation fails when tested by an appeal to the facts to be
observed at the present day, and we may confidently state that
the only source of origin which is at the same time adequate
and within the bounds of chemical and physical possibility is
terrestrial vegetation.
CHAPTER II
PBOCESSES OF FORMATION
IN the last chapter we have dealt with the material from which
petroleum is, or can be, formed, and the various theories that
have been put forward to account for its origin.
It now becomes expedient to consider the processes through
which the raw material must pass in order to convert it into
the mixture of saturated and un saturated hydrocarbons which
we know as "crude petroleum." The problem is to find out
whai: these processes are, and how they have affected the raw
material.
A simple distillation caused by heat will not meet the case
entirely. We have seen already that such distillations take
place in nature where igneous rocks invade coal or oil-shale
measures. Instances of this are frequent among the Scottish
oil-shales, and semi-liquid bitumen occurs as an impregnation
in porous strata or along joints and in cavities for some distance
from the shale bed or from the intrusion. But the result is
not the reproduction of an oilfield on a small scale, nor could
the process take place upon a sufficiently large scale.
What is required is a simple, slow, natural process which
can take place over wide areas. It is, without doubt, more in
the province of the chemist than of the geologist to make
investigations with the view of determining under what con-
ditions in nature it is possible to form petroleum from whatever
raw material is available; but the geologist's evidence is
necessary, if only to prevent undue attentkm being given to
entirely artificial conditions which may be arranged for in the
laboratory, but which can hardly be reproduced in nature.
Many chemists have conducted researches upon petroleum
with a view to proving its mode of origin and the processes
necessary for its formation, and no more careful and interesting
work has been done than by Engler and Hofer. These observers
25
26 OIL-FINDING
state very clearly the conditions under which the reactions
they observed and controlled took placo, and . the care and
accuracy of their researches cannot be doubted. But they do
not — and the same objection applies to the work of many
others on the same subject — approach the inquiry from the
point of view as to what conditions are possible in nature,
conditions which the geologist in however rough a manner is
able to define. Thus the work of these scientists, careful and
painstaking as it is, is open to the charge of what might be
called a form of special pleading in experimental work. Given
the conditions they postulate, the results are certain, but if
such conditions are practically impossible on a large scale in
nature, the researches conducted in a laboratory become of
little value to the practical man whose business is to find oil.
The geologist from his observation of the conditions under
which petroleum occurs, knows the conditions to which the
series of strata containing petroleum must have been subjected.
Some universal process, subject to these conditions, is called
for, and it is the duty of the chemist rather than of the geologist
to reproduce as far as is possible the conditions so defined, and
to prove whether it is possible to form the mixture of hydro-
carbons known as crude petroleum from the raw material
supplied and under the stipulated conditions.
Now the only conditions which the geologist has any right
to dogmatise about are depth-temperature, pressure, the presence,
or absence of water, the nature of the raw material, and the
question as to whether or not the strata in which the chemical
reactions take place have been sealed and isolated from the
introduction of extraneous material.
In the last chapter the nature of the raw material has been
discussed at length, and, so far as is possible at present,
determined. The calculation of depth- temperature is simple,
and within reasonable limits the temperature at which oil may
be formed can be deduced from incontrovertible evidence. The
calculation of pressure is a matter of much greater difficulty,
and there must necessarily be a very wide range between the
minimum and maximum pressures postulated. The sealing
up of the strata, in other words the determination as to whether
the reactions have taken place in open or closed retort, is again
a matter of easy determination, seeing that it is admitted by all
observers that for the formation or preservation of oil impervious
PROCESSES OF FORMATION 27
strata must overlie the petroliferous rocks. Similarly the
presence or absence of water, argillaceous material, sodium
chloride, and other material either active or inert in the
chemical sense, can be deduced with a fair degree of certainty.
Here we must turn to the laboratory to learn what ex-
perimental investigations will come to our aid ; it is a question
of conditions favourable to chemical reaction.
It has been stated that wood sealed in a closed tube with a
small quantity of water and subjected to great pressure at
ordinary temperatures has produced a small quantity of mixed
hydrocarbons analogous to a crude petroleum, but I have been
unable to verify this interesting result or to obtaiji any details
about the experiment. The various attempts, however, to make
commercial use of peat- masses furnish us with valuable evidence.
In Ireland, Sweden, the United States, and other countries, the
problem of how to utilize the enormous accumulations of peat
has for many years occupied the attention of practical chemists
and chemical engineers, and after many failures it seems 'that
some of the processes are within sight of commercial success.
Without disclosing information confidentially received it may
be stated that all these processes have this in common, that
the peat after being dried and perhaps ground and again
pressed into briquettes, is subjected to destructive distillation
in the presence of a limited quantity of water, under greatly
pressure, and at a comparatively low temperature.
The resulting products are various according to the end
aimed at and the different pressures and temperatures in each
case. Bituminous compounds, petroleum of almost every grade, "
and even coke may be obtained, while ammonium salts may be
recovered as sulphate by a process similar to that used in the
oil-shale and gas industries.
The important points for the geologist to note are that
petroleum of various grades and in great quantity can be
produced, and that the essential conditions are great pressure,--
cornparatively low temperature, and the presence of a limited ^
quantity of water.
Water is in any case present in the peat, even after drying,
for it is as impossible, without destructive distillation, to
remove the combined water in peat as it is in the case of a
lignite.
It is obvious that similar conditions can easily be obtained
28 OIL-FINDING
in nature. The presence of water in greater or less quantity
is almost inevitable in sedimentary rocks, the requisite pressure
is amply provided for by a covering of a few hundred, or it
may be thousand, feet of superincumbent strata, while as soon
as decomposition commences ' the potential gas pressure may
become so great that almost any hydrostatic pressure required
can be obtained. The temperature, increasing as it does on a
general average one degree Fahrenheit for every 55 feet of
descent into the earth's crust after the first hundred, would
soon be raised sufficiently to favour chemical reaction, while as
pressure increased the temperature would also rise till the
necessary equilibrium was reached. Thus once the process of
petroleum formation has commenced, its action is probably
automatic and must be complete, unless there is a change in
conditions. The sealing up of the strata by impervious rocks,
so that escape of gaseous or volatile compounds is entirely
prevented or rendered so slow and gradual, as to be quite
insignificant, is, as has already been stated, a question upon
which there is a general consensus of opinion.
It seems probable — but here we enter into speculation—
that it is the pressure that is the determining factor, as it is in
so many chemical reactions. Given the vegetable matter from
which petroleum can be formed enclosed in a well-sealed
deposit, given the presence of a limited quantity of water, and
the necessary, but by no means high, temperature, as soon as
the pressure reaches a certain point the action will begin. In
a deltaic area undergoing earth-movement, as is almost invari-
ably the case on the margin of a continent, sediment accumulates
very rapidly. A geosynclinal on a large or small scale, in fact,
is formed, and though sedimentation may occasionally outstrip
subsidence, or subsidence outstrip sedimentation, the general
result is the growth of the deltaic deposits outwards by pro-
gressive sedimentation over a continually increasing thickness
of strata belonging to the same series. In such circumstances
the requisite pressure for the formation of petroleum may
easily be obtained in the strata sufficiently deeply buried.
Another probable effect of pressure also must be con-
sidered ; ceteris paribus, the quality of the petroleum formed
is likely to depend upon it. In the process of isomerisation
of organic compounds, it has been proved over and over again
in the laboratory that pressure is usually the determining
PROCESSES OF FORMATION 29
factor. Thus a higher pressure may determine a more com-
plete condensation of the volatile compounds and gases into
light oils, provided that such condensation is accompanied by
a decrease in total volume. The fact that in many oilfields
where several separate sands at different depths contain
petroleum, the specific gravity of the oil generally decreases
as the depth increases may not be due in all cases, as has
often been assumed, entirely to partial and progressive inspis-
sation of the shallow oils, but partly to the pressure under
which the petroleum has in each case been formed.
On this hypothesis of oil- formation the importance of an
impervious " cover " also becomes apparent. The " cover " is
in effect the lid of the retort in which the chemical processes
take place. If the lid be imperfect or imperfectly closed,
escape of gaseous products must ensue, pressure can never
become very high, and the entire process of oil-formation may
be prevented, arrested, or permanently stopped. Coals or
lignites and carbonaceous shales and sandstones will be the
result. This accounts for the occurrence of porcellanite beneath
or forming part of a bed of shale or clay, while the lignitic
or carbonaceous phase is in evidence where the cover is
arenaceous and porous.
It has been suggested, on account of the association of oil-
bearing rocks with clays or shales often of great thickness, that
the argillaceous strata may have had some actual part in the
formation of the petroleum. This is a point very difficult of
proof, either for or against, since to bring actual evidence of the
favouring of chemical action by the presence of argillaceous
material which itself remains unaffected is well-nigh impossible.
It is quite probable that much of the material from which
petroleum is formed has been deposited with and included in
argillaceous sediment, witness the leaf beds which have been
burnt at outcrop to porcellanites. It is also certain, as proved
by Mr. Clifford Richardson, that clays can absorb and " adsorb "
bitumen to a remarkable extent, and can be used to filter
solutions of asphalt and asphaltic oils. But these facts are not
proofs of the argillaceous material taking any actual part in
the chemical processes by which oil is formed, even as what
used to be called a " carrier," a compound which, though itself
apparently unaltered, enables chemical action to take place by
continual decomposition and simultaneous re-formation. It is
30 OIL-FINDING
an interesting field for research for chemists to enquire into the
possibility of argillaceous strata having some such essential
role to play. For the geologist the matter of importance is
simply that potential oil-bearing strata require an impervious
cover if the oil is to be formed, and, when formed, if it is to
be preserved from inspissation, and that argillaceous rocks,
especially fine marine and estuarine clays and shales are the
best and most usual " cover-rocks."
By studying the subject of pressures in the earth's crust,
and by careful measurement of sections where oil-bearing strata
are exposed, it may be possible to arrive at some idea of the
pressure necessary for the formation of petroleum. In many
cases where large thicknesses of strata are exposed it will be
found that the lower part of the series is petroliferous and the
upper part carbonaceous, without there being any essential
change in the character of the intercalated sediments associated
with the oil-bearing and lignitic bands. It may be that the
upper part of the series has never been under sufficient pressure
to bring about petroleum-forming reactions.
Let us take a specific case and attempt, however roughly,
to calculate the maximum and minimum pressures which can
have been exerted during the formation of the petroleum. At
Point Ligoure on the western coast of Trinidad, where the
Guapo Oil Company is operating, there is a very clear section
exposing some 1300 feet of strata, the dip varying from
vertical at the northern and lower end of the section to 56
degrees at the southern and upper end. The lower 600 feet are
in the petroliferous phase, and several bands of oil-rock are
exposed especially near the base of the section. In the upper
200 feet of the section lignitic clays and sands with underclays
and thin seams of lignite are observed. In the lower part of
the section the strata are somewhat more highly mineralized,
concretions chiefly cemented with iron-salts are more frequent,
and there are several beds of fairly stiff argillaceous material
intercalated with the oil-bearing sandstones and above them.
In this case the mapping of the neighbouring districts has
proved that probably not more than 800 to 900 feet of strata
have ever been deposited above the uppermost beds in the
measured section. Assuming that such a total thickness of beds
has been deposited in a horizontal position, and again, assuming
that the pressure can be calculated as a hydrostatic pressure
PROCESSES OF FORMATION 31
directly due to the weight of the superincumbent strata — these
being great, and perhaps hardly justifiable assumptions — it is
possible to calculate the pressure to which the strata containing
the raw material from which petroleum can be produced have
been subject.
Taking the specific gravity of the strata to be on an average
2*7, we arrive at the result that the maximum pressure exerted
and applied in this instance has been 189 atmospheres, or some
one-and-a-quarter tons per square inch, and the minimum
approximately 135 atmospheres or rather less than a ton per
square inch on the strata now found to be oil-bearing, while
a pressure of 99 atmospheres was apparently insufficient to
determine the formation of petroleum. This calculation is, of
course, open to many sources of error, and it is improbable that
such high pressures have been exerted in this case, as earth-
movement and denudation probably prevented the accumulation
of any such thickness of strata in a horizontal position. The
figures are only given to suggest a form of enquiry in which
the observation of facts in the field may enable the geologist to
obtain evidence as to the conditions requisite for the formation
of petroleum. In this case the oil, as yielded at present, is
of fairly high gravity with an asphaltic base. Another instance
may be cited from a different region. In the valley of the
Yaw, in Upper Burma, an excellent section through the entire
Pegu Series of Burma may be studied, the total thickness being
some 8000 feet. The lower 3000 feet exhibit here and there
evidence of the petroliferous phase in seepages of a fairly light
oil with paraffin base, but lignitic beds begin to appear on the
same horizons as the oil-bearing rocks at about 3000 feet above
the base of the series. Then, after passing upwards through
some 1300 to 1400 feet of strata chiefly of solid clays, the
lignitic phase is well represented by a series of seams with
intervening underclays and sandstones, and up to the top of the
section no further evidence of petroleum is forthcoming. In
this case it is practically certain that earth-movement had
begun long before the deposition of the higher beds, and that
tho strata were never superimposed upon each other in a
horizontal position. Thus calculations of pressure and tempera-
ture from the data as given might be entirely erroneous. The
points to be noted, however, are that a transition from the
petroliferous to the carbonaceous phases takes place at a fairly
32 OIL-FINDING
definite horizon in the series, and that this change may not be
due entirely to the sealing up of the strata in which petroleum
is now found, but to 'a direct effect of different pressures.
Numerous other instances could be given, but these are
sufficient to suggest a field of enquiry which might be followed
up by laboratory experiments, the results of which might throw
light upon the conditions governing the formation of mineral
oils of every grade and nature.
Temperature. — The evidence as regards the temperatures at
which petroleum may be formed in nature is no less interesting.
It is evident that if depth-temperature alone is to be considered,
and in the case of most oilfields it is impossible to postulate
any other phenomenon capable of causing a rise in temperature,
there is no very great range of temperature available. In the
case of Point Ligoure a rise in temperature of 40 degrees
Fahrenheit would be all that could be granted, In the case
of the Yaw Valley it would not be safe to calculate upon a rise
in temperature of more than 52 degrees or 53 degrees.
Thus we see that the researches upon peat furnish an in-
teresting and attractive suggestion as to the conditions under
which mineral oils are formed in nature. High pressure and
comparatively low temperature are the conditions under which
petroleum can be produced from the vegetable matter of peat
masses, and similar conditions are at the least easily obtained
in the strata of what are now oilfields. The high temperatures
required for the destructive distillation of animal fats to form
distillates consisting of a mixture of hydrocarbons similar to
natural petroleum, are not only unnecessary, but can hardly be
assumed to be within the range of possibility.
Salt and Brine. — One other interesting and even puzzling
feature about many oilfields is the frequent association of
petroleum with brine or rock-salt.
The first oilwell drilled in America was intended to reach
brine and not petroleum, and in many other countries it has
been in the search for brine or salt that oil has been found. In
very many oilfields, also, the water associated with the petro-
leum or occurring in porous beds below it, and also frequently
above it, is brackish or even highly impregnated with sodium
chloride. In mud- volcanoes also, the water and mud discharged
are almost invariably saline.
PROCESSES OF FORMATION 33
It has been claimed that the occurrence of this brine is
confirmatory, in some unexplained manner, of the theory that
it is in marine strata and from marine organisms that petroleum
has been formed, and the well-known antiseptic properties of
common salt, under subaerial conditions, be it noted, have even
been adduced as being likely to favour the partial and selective
decomposition of animal matters which would be necessary if
petroleum is to be formed from them.
Into this speculation the author does not care to venture,
for lack of sufficient detailed evidence. But it must be admitted
that the terrestrial vegetation theory does not on the face of it
explain the presence of these saline waters, nor does their origin
from vegetable matter seem possible.
Without attempting an explanation, however, it is possible
to review such facts as bear upon the problem and to consider
how far these facts may indicate a possible solution.
In the first place it is necessary to ascertain whether brine
and petroleum are always associated or not; in other words
whether the former is an essential concomitant, or whether its
occurrence may or may not be due to causes not in themselves
directly necessary to the formation of mineral oil. Unfor-
tunately we are at present unable to answer this question with
certainty. In some oilfields a strong brine underlies or accom-
panies the oil in every petroliferous band, in most cases what
water is found is slightly saline or brackish, in a few cases
there is little evidence of salinity. In the famous Yenangyoung
field of Burma the waters met with in the upper oilsands, or
in water-sands between them, are fresh or only moderately
brackish, while a distinct brine has been struck in the lowest
sand penetrated up to February, 1911. In this case, however,
it may be that the upper waters have been briny and have
been diluted by the incursion of surface water. Thus the
percolation downwards of fresh water may result in the
occurrence of a small quantity of brine in the oilrocks being
overlooked.
Many oilfields contain regular beds of rock-salt, e.g. Luristan,
Persia and Texas, and these deposits may be found both
above and below oil-bearing strata. Again, in Persia brine
springs giving rise to saline rivers rise from some of the strata
which are approximately on the same horizon as oil-bearing
rocks in neighbouring districts. In the cases where brine is
D
34 OIL-FINDING
most conspicuous, a suggestive subject for enquiry is the in-
vestigation of the evidence as to the conditions under which
the strata now containing brine have been deposited, while it
is also necessary to take into account the present climatic
conditions under which the strata are observed.
In Persia, in the oilfields of Luristan, and more especially
in the strata overlying the known oilrocks, we have almost
every possible proof of a former desiccation during formation
Ked-coated inudstones and sandstones, deposits of gypsum on a
gigantic scale, Brockram-like breccias on the flanks of limestone
outcrops unconformably overlaid, are the rule throughout a
vast thickness; of strata. Furthermore there is indisputable
evidence of a contemporaneous earth-movement that shut off
basins and allowed the desiccation to take place. The occur-
rence of beds of rock-salt, therefore, can readily be understood,
quite apart from any suggestion of its being essentially associated
with petroleum. Furthermore, the climate of this region
(Plate IV) is very dry, absolutely rainless throughout a great
part of the year, so that there is no excess of surface waters to
dilute arid disguise the presence of brine in the strata. The
importance of this point concerning climatic conditions at the
present day can be appreciated when the logs of the wells
drilled in the Maidan-i-Naphtun field in Persia are studied.
Hardly any water has been encountered at any depth in any
of the wells. The significance of this point will appear
shortly.
In Baluchistan in the Khatan oilfield, a region almost
rainless, the waters associated with and accompanying the oil
are impregnated with salts, but instead of sodium-chloride it is
largely the sulphates of sodium and calcium that are present.
These salts occur frequently throughout great belts of the dry
zone, and are characteristic generally of arid regions, quite apart
from oilfields. Such evidence suggests that there may not be
any essential connection between the occurrence of salt or brine
and petroleum.
The whole question, however, requires exhaustive research
before it can be decided whether or no the oil and brine are
due to the same chemical action, whether they are different
effects of the same causes, or .whether their association is merely
adventitious. In the answers to these questions probably lies
one of the most illuminating generalizations yet to be made in
PROCESSES OF FORMATION 35
the geological study of petroleum, and one which may be of
great practical value to those who have to exploit new
oilfields.
What is required is a large number of analyses of the
brines and brackish waters found accompanying or underlying
the petroleum in an oilrock or discharged from a mud- volcano.
In each case it must be known from what depth the water was
obtained, with what particular kind of oil it was associated,
paraffin or asphaltic, high or low grade, whether sulphur
compounds were present in the oil, and if so, in what percentage,
and whether there has been any possibility of surface waters
having percolated downwards and mingled with the brine or
brackish water. Without precise data of this kind it is
dangerous to generalize.
The only suggestion that the author would put forward is
that it must not be forgotten that salt and petroleum may be
entirely unconnected. Every sedimentary rock — and many
igneous rocks for that matter — contains either sodium chloride
or ingredients which could furnish that salt if the rock were
sufficiently lixiviated. Where water is in excess, as in water-
bearing strata, the percentage of sodium chloride is so small as
to be inappreciable, but where water is in smaller quantity and
has percolated through a considerable thickness of strata it is
possible that a considerable concentration of saline matter in
solution may have taken place. Now we have seen that one
of the probable conditions under which petroleum has been
formed is the presence of a limited quantity of water. Much
of the hydrogen also may be utilized in the formation of the
mixture of hydrocarbons which we know as crude petroleum,
but this is very doubtful, as it would necessarily involve the
oxidation of any oxidizable material in the vicinity. Hpwever
this may be, it is evident that any residual water might
become a fairly concentrated solution of saline matter. As we
have seen that petroleum is formed in what we may consider a
closed retort, circulation of subterranean waters and percolation
of water from upper strata might be impossible or only possible
to a very slight extent, and a brine associated with the oil or
underlying it might survive without dilution till the oil-bearing
strata are pierced by the drill. The evidence of desiccation
In the strata overlying petroliferous rocks in many oilfields
shows that excess of water is not a probable condition in
36 OIL-FINDING
the series containing oil, for where rainfall is scanty and
evaporation rapid the absorption of water by the strata must
be minimised.
This hypothesis as to the reason why saline water is usually
found in association with petroleum is only put forward as a
suggestion, which must be tested by application to facts as
observed ; it is merely stated now as a guide to the direction in
which future research may prove profitable.
There is one other point in connection with the formation
of petroleum which cannot be too clearly insisted upon. It is
the common practice to distinguish between oils of asphaltic
base and oils of paraffin base, and they are often spoken and
written about as if they were entirely different minerals. In
some cases it has even been suggested that they have been
formed from different raw materials.
But there is actually no hard and fast line between asphaltic
and paraffin oil; many asphaltic oils contain a percentage of
solid paraffin, and many so-called paraffin oils can be made by
careful distillation to yield a residue of asphalt. In fact, there
is less difference between different crude petroleums than
between different coals, which, as is well known, show every
gradation from the least mineralized lignite with a high per-
centage of water, through bituminous coals and gas-coals to
anthracite, and, perhaps, finally even to graphite.
It has been shown that the light paraffin oils of Burma, with
percentages of solid paraffin up to as much as thirteen, and the
heavy asphaltic oils of Trinidad can both be proved to have
been formed from vegetable matter, while the paraffin oils of
Trinidad, with percentages of solid paraffin up to six (though
they occur under slightly different conditions from those
in which the asphaltic oils are found, in the former case
impregnating thin oilsands very well sealed up amidst
thick masses of clay), give no evidence of an essentially
different origin.
To account for the differences in grade and class of crude
petroleum, we must look to variations in the conditions of
formation ; different pressures are probably the most important
factors, but differences in temperature, relative quantity of
water present, and many other local conditions probably all
play their parts. In these questions there is need for much
research and experimental work in the laboratory, and it is
PROCESSES OF FORMATION 37
hardly within the province of the geologist to speculate upon
the effects of the environment to which the raw material was
subjected. It is, however, the geologist's task to deduce and
discover as far as possible what that environment must have
been, so that armed with the knowledge thus gained the
chemist's task may be simplified.
CHAPTER III
THE MIGRATION, FILTRATION, AND SUBTER-
RANEAN STORAGE OF PETROLEUM
IT is necessary now to consider what may happen to the crude
petroleum after it has heen formed, what movements are
possible for it, and the reasons for those movements, how it is
concentrated and stored, and how it may be affected in grade
or quality by the conditions to which it is subjected. The
migration, filtration, and storage of oil in nature are subjects
so inextricably connected that they can hardly be considered
apart ; they must all be understood by the geologist if he is to
be capable of reading field evidence correctly and assigning its
true significance to every indication which he may have to
consider of the presence of petroleum, at the surface or in
a well.
The causes for the migration of oil are earth-movement,
hydrostatic pressure, and gas pressure. There are many factors
which determine movements of oil, but directly or indirectly
all movements are due to these three causes. The theory that
oil is underlain by water or brine and has been floated up by
the heavier liquid through porous strata, and thus by the
hydrostatic pressure of the water forced towards the crests of
flexures or to outcrop, is pretty generally accepted, and certainly
in fields such as those of the Eastern States in America, where
the strata often lie at low angles over great stretches of country
with very small and gentle flexures and disturbances, and the
porosity of the rocks does not vary sufficiently to hinder
migration, there may have been a great lateral progression of
petroleum towards the localities best adapted for storing it.
But cases are not always so simple, and to assume that in any
oilfield the petroleum contents have originated at a great
distance, and have only reached their present position after a
wearisome journey, is quite another matter. The insistence
38
THE MIGRATION OF PETROLEUM 39
upon the migratory feats of petroleum has arisen to some extent,
at least, from the desire to account for the formation of the
hydrocarbons from animal matter. Thus, on the theory that
the oil of the Californian and Texas- Louisiana fields has been
formed from the soft parts of foraminifera preserved in thick
masses of shales and clays, it is necessary to postulate a migra-
tion of each minute particle through almost impervious strata
in a certain direction to form an accumulation in a porous
stratum. To attribute such a movement to the hydrostatic
pressure of water is perhaps to attach too great importance to
an action which in porous and inclined strata does without
doubt take place. But it has already been shown on what very
doubtful evidence a foraminiferal- origin theory rests. If on
the contrary the oil is formed from accumulations of vegetable
matter, it is not necessary to postulate extensive migration as
a rule ; strata capable of containing the petroleum are usually
at hand, and in these strata it will be found. The Tertiary
Series in Burma and Trinidad, where great thicknesses of strata
of estuarine origin are present, supply abundance of evidence
on this point, while lignitic or carbonaceous beds contempora-
neous with the oil-bearing strata, and at no great distance from
them give evidence of the presence of the raw material, and
suggest that no great or extensive migration is necessary.
Hydrostatic Pressure. — It is the geological structure and
the porosity of the oilrocks that determine the effects of
hydrostatic pressure. The rocks must be sufficiently porous
to admit of free, if slow, movements of the aqueous contents,
and the strata must be sufficiently inclined to determine the
direction of movement. Thus towards the crests of anticlines,
both laterally and upwards, there must in nearly every case
be a gradual migration of oil by the gradual replacement by
water in the lower levels, when there is a sufficient difference
in the specific gravities of the liquids. In a subsequent
chapter the various structures that favour such migration will
be dealt with.
The question of specific gravity becomes in some fields a
matter of great importance. The fact seems to have been lost
sight of occasionally that a heavy asphaltic of say 0*95 specific
gravity or higher will be affected much more slowly than a
light oil of 0*72 specific gravity. Consequently in considering
lateral or upward movements of petroleum the particular
40 OIL-FINDING
grade of the petroleum must be taken into account. To over-
come the friction and the viscosity of the oil which must
necessarily retard percolation, a considerable advantage in
specific gravity must be possessed by the water. Thus to
generalize on the subject of migration of oil from facts ascer-
tained in the Pennsylvania fields, where a light paraffin oil is
found, and to apply the generalizations to such fields as
those of California, or even Baku, where an asphaltic oil of
heavier gravity is the rule, is, to say the least, very unsafe.
Gas Pressure. — Another cause of what may properly be
called migration of oil is gas pressure. The gas may not
exist as such in the strata, being dissolved . and occluded to a
great extent in the petroleum, or the pressure may be too
great to allow of the existence of gas if it is below the
" critical temperature." In that case the gas will be in a
potentially gaseous state, and must exert an enormous
pressure in seeking to find space in which to expand to the
gaseous state. The terrific force with which such gas is dis-
engaged on the striking of a prolific well is sufficient evidence
on this point, as it is now admitted that gas pressure is the
chief if not the sole cause of fountains or flowing oilwells.
This gas, dissolved, occluded in or mechanically associated with
the oil must exercise pressure in all directions, and here again
comes a point that has frequently been lost sight of. It is
often assumed that the movements of gas and oil must be
directly or indirectly upward, and this has often caused
deplorable errors to be made in the location of oilwells and
in the deepening of wells long after they have passed through
the lowest strata in which there is any hope of oil being struck.
A " show " of gas in a well has only too frequently been
understood as a sign that oil must lie beneath.
But if gas exerts pressure in all directions it will migrate
in all directions till stopped by some impervious stratum.
Thus both laterally and downwards there may be a migration
of gas carrying with it probably small quantities of the lighter
constituents of the oil. The oil will gradually be trapped
during the migration, especially by argillaceous strata, so that
the gas finally reaches furthest from the parent source. It is
owing to this that we find gasfields spreading beyond the
confines of an oilfield, and profitable productions of gas may
be. obtained near a prolific oilfield but in localities where no
THE FILTRATION OF PETROLEUM 41
oil can be struck and where the strata may be substantially
waterlogged. Instances of this are not uncommon in Burma.
Again, gas may be found beneath the oil-bearing strata,
and may be evolved from clays and other almost impervious
rocks long after the porous oil-bearing strata above have been
removed by denudation. The impregnation of strata uncon-
formably overlaid by oil-bearing rocks has been observed in
many parts of the world ; good instances are recorded from
Alaska, where metamorphic rocks have been impregnated from
the Tertiaries above them, and from Galicia, where Cretaceous
strata contain oil derived from, an overlying Tertiary Series.
Such cases of impregnation have also come under the writer's
personal observation in Baluchistan and Trinidad. In Burma
also there is some evidence of deep wells passing through the
petroliferous Pegu Series and striking oil in the unconformable
series beneath. In some of these cases the strata beneath are
argillaceous, so that they contain little more than gas, and
perhaps a little filtered oil, the exudation of which when
exposed at the surface is naturally very slow.
In the south-eastern corner of Trinidad slow evolutions of
gas may be seen from an outcrop of clay of the Cretaceous
Series, which is not petroliferous in the district, but which is
overlaid unconformably in the immediate vicinity by oil-bearing
Tertiary sands. In the Piparo district of the same island the
discharge of gas in one locality has been sufficient to form two
small mud-volcanoes on an outcrop of Cretaceous clay which
was not originally petroliferous. In this instance, however,
the volcanoes may be fed from some more porous strata
beneath, which have been more completely impregnated from
the Tertiaries.
Filtration Effects. — Any oil appearing with the gas in such
cases will probably be well filtered and to a large extent
decolorized. Professor Clifford Richardson has proved that
by continued filtration through clay solutions of asphalt and
petroleum of any kind may be almost completely decolorized
owing to the absorptive and " adsorptive " properties of the
clay. The fraction " adsorbed " cannot be extracted again by
treatment with solvents, and so is distinguished from that
absorbed. This phenomenon is very suggestive, as similar
conditions may easily be reproduced in nature. Where oil
is obtained from argillaceous rocks it is almost invariably
42 OIL-FINDING
light in gravity and colour, and productions are not as a rule
large nor gas pressure great. The water-clear oil of Kala-
Deribid in Persia (Plate V) is the most striking instance that
has come within the writer's observation. This oil, which is
perfectly " water-white," collects very slowly in small holes
dug in the outcrop of a fine-grained, compact shale, exposed
in a small stream valley. There is very little evolution of
gas in this case, only a few bubbles being noticed, as compared
with the brisk evolution so frequently observed from an outcrop
of oilrock. The " show," though on the crest of a large and
sharp asymmetrical anticline (Plate VI), is not concentrated
towards the actual line of crest, but distributed for some 20
or 30 yards through the outcrop of the shale on the gently
dipping flank of the flexure. This surface indication, in fact,
differs essentially from the usual show of oil on an anticlinal
crest ; the petroleum does not seem to be forced up or carried
up by gas, but collects particle by particle, just as water collects
in an excavation in a water-bearing sand. The greatest yield
is about four kerosine tins per day.
Close above the shales occur several outcrops of rather
loosely compacted sandstone, which have all the appearance of
weathered oilsands, but which, beyond traces of sulphur,
contain no sign of oil. Such traces of sulphur are often the
last surviving evidence (in a thoroughly lixiviated sand) of the
former presence of oil which contained sulphur.
The author's theory with regard to this water-clear oil is
that it is a filtered residue yielded slowly by the almost
impervious argillaceous rock, that we must look for its origin
in oilsands lying above the shale, and that it affords an instance
of downward migration of oil, only the filtered remains of which
have been preserved by the less easily weathered shale.
Filtered oils, varying in colour from water- clear to that of
a well-matured brandy, which are obtained in small but payable
quantities from shallow wells in the limestone of Eamri Island
off the coast of Arakan, have probably a similar origin. The
yield is steady and slow, the gas pressure small, while inspis-
sation has, as in the case of Kala-Deribid, removed most of the
more inflammable fractions, giving the oil a high flash-point,
and enabling it to be burnt in ordinary lamps without distil-
lation. In this case also the overlying series is petroliferous,
and otlshows on a large scale with explosive discharge of
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THE FILTRATION OF PETROLEUM 43
gas from younger strata are not far distant, e.g. Faule
Island.
In Baluchistan a somewhat different case of migration into
older strata may be observed. The impregnation is only along
joint planes and in beds of slightly greater porosity in a
compact limestone, while the oil is a dark heavy residue
containing sulphur and very little light oil. The shows occur
on the flanks of a range of hills formed of limestone, anticlinal
in structure, and overlaid by a thick series of shales. It is
only at the edge of or beneath the outcrop of the shale that any
appreciable production of oil has ever been obtained (e.g.
Khatan), and the oil is very heavy, contains a large proportion
of sulphur compounds, and is accompanied by warm sulphur
springs. It seems perfectly clear that we are dealing with the
inspissated residue of a partial impregnation which took place
before denudation had laid bare the series so deeply, and that
now only the ail-but final results of inspissation are in evidence
to indicate that impregnation of lower strata has taken place.
The overlying shales are in places distinctly bituminous, and it
is from their outcrop not many miles distant that a bituminous
coal rich in pyrites is mined.
A more striking instance of much the same phenomenon,
and one that can be more easily and completely studied, is
afforded by San Fernando Hill in Trinidad. The hill is formed
of an inlier of a peculiar rock called "argiline" by Messrs.
Wall and Sawkins, and it forms the core of an anticline in the
petroliferous Tertiaries which overlie it. The argiline, though
an exceedingly fine-grained rock, has been impregnated through-
out, and owing to its homogeneous nature and closeness of
grain has been enabled to retain the impregnation under
weathering influences for a considerable time. In the numerous
quarries opened in this argiline, a crust usually some six to
eight feet thick of the weathered material is observed, separated
sharply from the part still impregnated ; the line between
weathered and un weathered argiline crosses the bedding obliquely
in many places. At the north-eastern end of the hill sticky
inspissated oil has exuded in considerable quantity, so much so
that a syndicate was once formed to work it, but after doing
some excavation the enterprise was abandoned as unprofitable.
Similar attempts are often made to obtain oil where some such
deceptive " show " has tempted men of enterprise, but without
44 OIL-FINDING
geological knowledge, to commence development work, arid it
is largely from such unsuccessful attempts that the popular
idea of the great uncertainty of oil exploitation has arisen.
Another instructive example is furnished by the first well
drilled by a company now operating in Trinidad. The well was
commenced below the horizon of the oil-bearing rocks of the
district, and, after passing at shallow depth through strata with
slight indications of oil, entered a thick series of clay from,
which a certain quantity of gas issued. This gas assisted to
puddle the clay, which caved badly, and made it rise in the
bore-hole and thus cause great difficulty in the drilling. The
clays at this horizon are of great thickness and only contain
small, inconstant, and insignificant beds of oilsand. After
struggling for months with these argillaceous strata and the
gas, the well was abandoned, having reached a depth of only
some 500 feet. It was probably the occurrence of gas that
induced the company to persevere with the drilling, although
they had been warned before the derrick was erected that the
geological sequence of strata had been worked out carefully, and
that the well would certainly prove a failure.
These instances are merely quoted to show of what practical
importance it is that the probability of downward migration of
petroleum and gas, even into almost impervious strata, should
be recognized.
Another theory that is sometimes expressed regarding migra-
tion of oil is that it has been present in some particular area,
but has escaped, by means of faults in the strata, and so is no
longer available nor can be struck in a well. This is one of
the many suggestions made about faults by those whose personal
or practical acquaintance with geological work is small, but
who make use of the idea of faulting as a sort of deus ex
machina to account for something which they do not under-
stand or have been unable to explain. It is reminiscent of an
antique method in geological mapping, the observer when
involved in serious difficulties boldly mapping a theoretical
fault and starting afresh. In books on the subject of petro-
leum, when faults are mentioned the word is usually followed
by the words " fissures " and " crevices." " Crevice," by the
way, is a favourite word with the careless driller who has
provided himself with a " fishing-job," and who lays the blame
for the disaster upon a " crevice," which, suddenly entered
THE FILTRATION OF PETROLEUM 45
upon, caused too great a strain to be put upon some part of
his string of tools or cable.
Faults, fissures, and crevices are stated to have considerable
effects upon the underground storage of petroleum by affording
channels which allow the oil to escape upwards, downwards, or
laterally, and to have disappeared from the rock in which it
was stored.
Let it be admitted at once that faults do not infrequently
affect oilfields either favourably or unfavourably, and have often
a notable local effect in increasing production. Their effects
are purely structural, and will be dealt with in a subsequent
chapter on geological structure. As channels for migration of
oil to any important extent they do not act, for the simple
reason that a "fault- fissure" as the term is used in geological
parlance is not an open fissure in the ordinary sense of the
word. Open fissures are in any case very rare in nature, and
only occur in limestone formations or in hard igneous or meta-
morphic rocks, and then usually comparatively near the surface.
Were any open fissure to be formed in soft Tertiary strata, the
pressure would be sufficient to close it very rapidly, while if
petroleum were to commence migrating by such a channel the
fissure would soon be clogged by inspissating oil and the sand
or clay brought with it. The storage of petroleum in any
locality necessitates a more or less impervious cover, usually
of considerable thickness, and this covering would require to
be completely dislocated, a fissure opened and prevented from
becoming sealed before there could be any possibility of the
escape of petroleum in quantity. Where the covering is largely
of soft argillaceous strata such a phenomenon is manifestly
impossible. Another point also falls to be considered; even
with an oil-bearing series entirely exposed at outcrop, the
petroleum contents are dissipated by exudation at the surface
very slowly. At the depth of some hundred or even thousands
of feet such action into a narrow open fissure could not but
be very gradual.
Where one does obtain evidence of a form of migration to
which the expression " intrusion " may be applied, is in veins
of manjak, which term is used to include gilsonite and its con-
geners, and ozokerite. Manjak is to an asphaltic oil what
ozokerite is to one of paraffin base. They are inspissated oil
in veins which have actually been intruded usually in a vertical
46 OIL-FINDING
or highly inclined position from oil-bearing strata below, and
the material has consolidated without reaching the surface.
Occasionally such veins may be found along lines of fault, but
all those with which the writer is familiar are either along
bedding-planes or along minor slip-planes and joints in thick
masses or argillaceous strata. The phenomena associated with
manjak veins will be dealt with more fully later ; the point to
be noted at present is that if petroleum did migrate to any
extent along fissures, cracks, or fault-planes, we should find
abundant evidence of its having done so in veins of manjak
or ozokerite. But these phenomena, though known in many
parts of the world, cannot be said to be common occurrences
in oilfields, while faults are frequent to a greater or less extent
everywhere that earth-movement has been in operation, and
there is hardly an oilfield that is without some evidence of
faulting.
From all these considerations it will be seen that the
migration of petroleum is a very circumscribed action, and
cannot be called upon to explain any very widespread
phenomena in oilfields. To put it briefly, petroleum goes
where it can, but from the very nature of the conditions under
which it has been formed and under which it is preserved
its migrating movements are checked and hindered in almost
all directions. Thus when earth-oils are discovered in any
locality, we are almost justified in applying to them the famous
conclusions of the gentleman who devoted his life to research
upon the subject of the " fiery flying serpents in the wilderness,"
with special attention to their origin and subsequent history :
(1) " They was there all the time," and (2) " they stayed where
they was."
Subterranean Storage. — The relative porosity of strata is one
of the determining factors in the movements of oil, and the
selection of a reservoir rock. Oil will find the nearest available
porous strata and will impregnate them. Given sufficient time
and pressure it will impregnate, and even to some extent force,
its way through, an apparently impervious clay, but it will
select the most porous stratum to impregnate. This is the
reason that a gas-show, with a slight show of light and perhaps
light-coloured oil, is so often struck in a well some little
distance above the main oilrock. It is a filtered oil which has
gradually accumulated in a porous band, after passing through
SUBTERRANEAN STORAGE 47
the almost impervious cover of the true oil-bearing stratum.
In most cases, however, it is only gas that is found under these
conditions.
The great majority of oil-bearing rocks are arenaceous, sand-
stones of all kinds, grits or conglomerates, but some of the
world's most famous oilrocks are limestones and dolomites.
In the case of calcareous rocks it is probably merely because
the limestone affords a porous reservoir that it is found
impregnated with oil, just as in a manjak mine a nodule or
nodular band of ferrous and lime carbonate, being slightly
more porous than the surrounding clay, will contain more
evidence of petroleum than the country rock. However, as the
occurrence of oil in limestones has been made use of as an
argument in favour of the animal-origin of petroleum, it is
necessary to examine the evidence carefully. The famous
Trenton limestone of North America is perhaps as good an
instance as could be chosen. It contains barren areas and areas
of partial impregnation, as well as areas where great productions
of petroleum can be obtained, and it has been the subject of
much research. It has been proved that in the localities where
the rock is most productive it is cavernous in structure, con-
taining innumerable small cavities which are often drusy, and
which are found full of oil. Analysis has shown that the
cavernous variety of the Trenton rock is dolomitized, the
dolomitization naturally causing an increase in specific gravity,
which connotes a decrease in volume, and thus causes the
cavities. The rock, in fact, over wide areas, though sometimes
only on selected horizons, has been formed into what may be
compared to a sponge, and the oil contents vary in quantity
directly with the degree of dolomitization. It is difficult, if
not impossible, ta imagine any chemical action which will
bring about the dolomitization of a limestone and at the same
time produce petroleum, and one is forced to the conclusion that
the presence of the oil is accidental, and that it has occupied
the cavernous limestone simply because the rock afforded room
for it. Much of the impregnation, by the way, may be due
to downward migration. In this connection we may bring
forward confirmatory evidence from the cavernous limestones
of Maidan-i-Naphtun and Marmatain in Persia. These are
the oil-bearing strata from which the Anglo-Persian Oil Company
is obtaining such remarkable productions, and they were first
48 OIL-FINDING
studied in detail by the writer, They are grey porous lime-
stones, containing innumerable small cavities, generally lenti-
cular in shape and attaining to a diameter of as much as one
inch. The cavities are frequently drusy ; their presence makes
the rock in bulk exceedingly porous.
Careful mapping of the Maidan-i-Naphtun field proved that
these are not ordinary limestones, but are of detrital origin ;
they vary from very coarse breccias consisting of large irregular
blocks of limestone and sandstone in a calcareous matrix on
the one hand, to thin calcareous sandstones and finally ordinary
sandstones on the other. The thinning out of these calcareous
masses, becoming sandier and occasionally more argillaceous as
they thin out, is beautifully seen. Surface indications of oil
occur in these strata even where they have thinned out into
bands of sandstone a few feet thick, but the "shows" are
greatest where the different bands coalesce into thick masses
of calcareous rock. The origin of the limestone fragments,
which are often most irregular, is the Cretaceo-Eocene limestone
of Asmari, a very thick calcareous formation which is overlaid
unconformably by the Tertiary petroliferous series.
These limestones, being of detrital origin, cannot be brought
forward as evidence of the animal- origin of oil, as has been
done in the case of the Trenton rock. Yet they present the
same cavernous and drusy characters. Analysis to determine
whether doloinitization has taken place or not has not yet been
undertaken, but the rock has all the appearance of a dolomite,
and the writer has little doubt about the matter; the drusy
cavities certainly contain crystals of dolomite. The conclusion
is obvious : the cavernous rock has become impregnated with
oil, because it is the most porous reservoir available amidst a
thick series of gypsum, shale, and mudstone beds.
At Jemsah, on the Gulf of Suez, very similar strata contain
the oil-bearing beds, which are again cavernous dolomites.
Spindle Top gives another instance of a cavernous limestone
or dolomite containing petroleum in quantity. In this case
the impregnation is probably due to lateral migration aided by
earth-movement.
It is perhaps not out of place to mention here that lime-
stone oils frequently exhibit some differences from sandstone
oils, and though those differences may not be essential, they may
be of considerable practical importance. Thus many limestone
SUBTERRANEAN STORAGE 49
oils are noted for the percentage of sulphur which they contain ;
their outcrops are often marked by sulphur springs and evolution
of hydrogen sulphide, while crystals of pure sulphur may be
found lining cavities in the oilrock. Spindle Top, Marmatain,
and Maidan-i-Naphtun in Persia, and Khatan, Spintangi and
Kirta in Baluchistan are instances. In these cases there is
reason to believe that the sulphur compounds may not be
entirely original in the petroleum, but may be due to the
action of the oil and water on sulphides contained in the strata.
In oilsands and their associated clays, pyrites and marcasite
are not uncommon, but in the limestones of the above men-
tioned oilfields these minerals are apparently absent. It is
possible that the petroleum may have absorbed and incorporated
sulphur compounds encountered during its migration to and
through the limestone which it now occupies. In the cases of
Khatan and Spintangi the shales where the oil originated are
full of pyrites in the area where the carbonaceous phase is in
evidence, and the Harnai Valley Coal, as the bituminous coal
worked in these shales is called, contains a large quantity of
pyrites. This is absent at Khatan, but sulphur compounds are
present in the oil, and sulphurous springs appear every here
and there from beneath the outcrop of the shales.
Parallel evidence can be obtained within the confines of
Great Britain ; in the west of England where the Carboni-
ferous limestone becomes slightly bituminous in some locali-
ties, the foetid odour of a fresh fracture gives unmistakable
evidence of the presence of hydrogen sulphide.
It is not suggested that sandstone oils do not contain
sulphur compounds, many of them are unfortunately very rich
in this, in oil, undesirable element, but there seems to be some
condition affecting oils enclosed in limestone which makes it
possible to decompose any sulphides present and to incorporate
a percentage of the sulphur in the oil, which percentage naturally
becomes more conspicuous as the sulphur compounds are con-
centrated by the inspissation of the petroleum.
Further research is necessary upon this point, but the
suggestion of the effect of environment on the oil after its
formation is made here as it may be of practical utility. The
same oil that impregnates a limestone in one locality, where
it is associated with abundant evidence of the presence of
sulphur, may be found in a locality not far distant impregnating
E
50 OIL-FINDING
a sandstone, and containing a smaller percentage of sulphur
compounds and consequently being of higher quality and
better value.
It is to sandstones, however, that we owe our principal
supplies of petroleum, and almost every variety of sandstone
may be found acting as an oil reservoir.
Here one of the popular ideas of the driller may be sum-
marily dealt with. In oilfield work one is frequently informed
that there are oilsands, gas-sands, and water-sands, and that they
have essential and different characteristics, while some informants
will even go so far as to state that they can tell by examination
of a clean sample of sand to which of these three classes it belongs.
These are men often of acute observation, and they may be per-
fectly right for a particular field, or in a particular locality, but
to generalize, in one or even several oilfields from the evidence,
and to expect the generalizations to prove true of another field,
perhaps in a different country, is notoriously dangerous. True,
in one field the oil-bearing horizons may be composed of a
certain kind of sand of characteristic coarseness, colour, contour
of grain, and porosity, while gas or water may be found in the
same locality in arenaceous strata of different types, but a sand
m ly ch an ge almost entirely in character within the space of a
few hundred yards, and yet remain none the less an oil sand,
gas-sand, or water-sand as the case may be. Proceeding down
the flank of an anticlinal flexure, or down the pitch of a dome
structure, what was the oilsand near the crest may be found
destitute of oil and full of water.
Again, some sandstones, especially those with calcareous
cement, may be so compact as to be hardly capable of con-
taining appreciable quantities of oil, but may contain gas. But
when one considers the conditions under which such sands have
been deposited, it becomes obvious that the calcareous cement
may vary in quantity in different localities, and the porosity
of the rock may vary as much. Thus the same sandbed may
be rich in oil at a comparatively short distance from where it
was merely a gas-sand. A low-lying shore, such as may be
seen on the eastern coast of Trinidad, is an object lesson in
arenaceous deposition. There every gradation from a shell-
bed, formed almost entirely of fragments of broken shells, to
a pure siliceous sand, a muddy sand or a sand containing
vegetable matter, which will eventually be a carbonaceous sand,
SUBTERRANEAN STORAGE 51
may be seen accumulating under the action of waves and
tides. Each variety will differ from the others in size and
contour of grain, chemical composition and porosity, and all
are being formed simultaneously within a distance of, perhaps,
less than a mile of coast line.
It cannot be too clearly stated and understood that an
oilsand is a sand containing oil, a gas-sand one containing gas,
and a water- sand one containing water. Remove the contents,
and they are no longer entitled to the names, though they may
still be mapped geologically and designated as the horizons of
such and such oil-, gas-, or water-sands.
Contour of Grain. — On the subject of the contour or shape
of the grains in an oilsaiid there has been some confusion of
opinion. Mr. A. Beeby Thompson in his book on the " Oil-
fields of Russia " gives microphotographs of sands from Baku
oilwells, calling attention to their fairly well-rounded character,
on the strength of which he suggests that they are wind-blown.
On the other hand Professor Clifford Richardson in his book
on " The Modern Asphalt Pavement " gives microphotographs
of the sand extracted from the asphalt of Trinidad's famous
Pitch Lake and washed, calling attention to the sharpness of
the grains, on the strength of which he suggests that the
silica has been deposited from solution. Now the sand grains
in Trinidad asphalt from the Pitch Lake are derived, as is the
bitumen, from the La Brea oil-bearing group, on the outcrop
of which that great asphalt deposit has been formed. Here
then are two authorities who have examined the silica grains
from different asphaltic oilsands, one calling special attention
to the roundness of grain, and the other to the sharpness. This
is quite sufficient to prove that oilsands differ considerably in
the matter of the shape and contour of grains, but in this
particular case the writer is unable to agree with either Mr. A.
Beeby Thompson or Professor Clifford Kichardson on the
evidence which they have brought forward and figured. The
Baku sands are neither so well-rounded nor so evenly graded
as typical wind-blown sands; it is more probable that any
special degree of smoothing or rounding which these grains
exhibit is due to the attrition they must necessarily have
experienced in the well before being brought to the surface,
In a flowing oil well, where sand is brought up with the oil,
there must be a great churning up of the siliceous material,
52 OIL-FINDING
quite sufficient to add a polish to the grains. Mr. Thompson's
further suggestion that the sharpness of these grains may have
caused an epidemic and rapidly fatal disease among shoals of
supposititious fish, the carcases of which provided the raw
material for the formation of petroleum, hardly bears out his
contention as to the sands being wind-blown.
In the case of the La Brea oilsand, the grains are certainly
neither so sharp nor so distinctly broken fragments of crystals
as to suggest deposition from solution. It is a very ordinary
water-borne sand.
There is, however, no reason why wind-blown sands or any
other kind of sand should not become impregnated with oil:
any porous rock will serve.
Porosity. — Porosities naturally vary very greatly in oil-
bearing strata, and as it is on the thickness of an oilrock and
its porosity that production ultimately depends, the subject is
worthy of careful study.
The voids in a rock may be as much as 40 per cent, by
volume, but that is exceptional and unlikely to be met with.
Percentages of 20 and 25 of voids, however, are not by any
means rare occurrences in sands, and given that the voids
are completely filled with oil, a prolific production may be
expected. Too porous an oilsand has its disadvantages, since
on being struck in a well the cohesion of the stratum is liable
to be completely broken down, and quantities of sand brought
up with the oil. This may cause choking of the casing, the
wearing away of flow-heads and caps put on to check or
control the flow, and in extreme cases the derrick may even be
half buried in sand.
In Baku the quantity of sand brought up by flowing wells
has been a cause of great trouble and expense, and in California
and Trinidad similar difficulties have been encountered. The
oilsands of the latter colony have been analysed by Professor
Carmody, Government Analyst and Director of the Agricultural
Department, samples being taken from outcrop for the purpose.
Percentages by weight of from 15 to 18 of inspissated petroleum
have been recorded from outcrops of the Eio Blanco oilsand.
By volume this would mean nearly three times as much, so it
may readily be understood that in some cases the surface of the
outcrop actually shows signs of flow. Strata in this case cannot
be broken by a hammer, being too soft, but small fragments
SUBTERRANEAN STORAGE 53
can be twisted off in the fingers and rolled into pellets. Where
the oil is not inspissated the percentage both by volume and
weight will not be so high, but it is evident that strata so rich
in oil will break down easily when struck in a well. The
experience of those companies who have drilled into the Rio
Blanco oilsands is that sand is always tending to fill up part
of the casing, and the wells must be constantly cleaned if their
production is to be maintained.
The La Brea oilsands, the youngest oil-bearing rocks of
Trinidad, also contain a very large percentage of petroleum,
and it does not require the experience of drilling near the Pitch
Lake to prove that the cohesion of the rock is very easily
broken down ; the Lake itself is sufficient evidence. The
winning of oil from wells drilled to this oil-bearing group will
always be subject to great difficulties on account of clogging
of the casing by heavy oil and sand, and the wells will require
constant cleaning out.
The most satisfactory oilsands are those which are sufficiently
compact to maintain their cohesion even when the well is
flowing. The greatest productions are not obtained from such
sands, but wells drilled into them have a longer life and are
worked much more economically.
Paraffin oils, perhaps because they are as a rule of lighter
gravity than asphaltic oils, seem to disengage themselves more
easily from the sands, and do not, as a rule, carry so much
sand with them. This, however, may be partly due to incipient
paraffin ation, the deposit of paraffin scale in the sand helping
to maintain the cohesion of the rock. The sudden relief of
pressure and consequent lowering of temperature when a prolific
well is brought in in a paraffin-base oilsand must cause solid
paraffin to be deposited. If a well in such circumstances be
not carefully looked after, its life maybe shortened considerably
by the sand near the bottom of the bore-hole becoming com-
pletely clogged with solid paraffin.
The great advantage that limestone has over sandstones as
an oil reservoir is that it does not break down and choke the
bore-hole, and another advantage of almost equal importance is
that it is possible to torpedo a limestone well, the production of
which has fallen off badly, by exploding a charge of nitro-
glycerine at the bottom of the bore. This usually results in
giving the well a new lease of life. It -is seldom of any
54 OIL-FINDING
use to torpedo a well in sandstone, even if the rock be fairly
hard.
Oil occurs not infrequently in shales and clays where they
have some degree of porosity, but the yield of wells drilled
into such strata is always small and the petroleum accumulates
very slowly. The oil is naturally well-filtered, and light in
such conditions, but production is seldom sufficient to ensure
a commercial success. In Java wells have been drilled into oil-
bearing shales, yielding an excellent oil, but not in sufficient
quantity.
There is some evidence suggesting that an oil may, under
certain conditions, prepare its own reservoir by the removal in
solution of cementing material in a rock. This probably applies
only in the case of calcareous cement, and may take place only
within the zone of weathering, but as that zone may extend
downwards for some hundreds of feet the results might be
important.
On the southern coast of Trinidad there are many sections
where the cliffs have been cut back by marine denudation,
leaving a very gentle slope of clays usually much land-slipped
— a plan, in fact, of former landslips. The strata dip steeply,
and contain numerous thin beds of calcareous sandstone which
stand out in lines above the clay surface, but are often dis-
continuous.
These sections, though washed twice a day by the tide, reek
with the odour of petroleum, and a close examination shows
that similar small reefs of brown oilsand are contained in the
clay. These oilsands are seldom more than a foot or two thick,
and they resemble the calcareous sandstone reefs in every way,
except that they contain little or no lime, and are very much
softer and consequently less prominent. They are quite full
of petroleum which exudes steadily and slowly, forming films
upon the pools of water left by the receding tide. The oil is
light, and is accompanied by very little gas.
Washed about on the shore, and sometimes embedded in
the clays near the small reefs of sandstone, are large botryoidal
masses of calcium carbonate. These masses are dark in colour
owing to the inclusion of a proportion of clay, but the calcite
is well crystallized, the crystals radiating from the centre of
each rounded mass. These botryoidal masses are quite different
from the ordinary fine-grained calcareous concretions of the
SUBTERRANEAN STORAGE 55
clay. They occur in many parts of the island, but always near
the outcrops of oil-bearing strata. Unfortunately, they are
generally found loose, washed out of the clay.
In one locality near Galfa Point in the Cedros district, a
bed of sandstone some six feet thick is exposed among clays
on the foreshore. Part of it is hard and calcareous, and part
comparatively soft, brown in colour, and highly petroliferous.
In the calcareous portion petroleum is only seen along joints
and bedding planes. The calcareous cement does not occur
like a concretionary mass, but is quite irregular in outline and
appears as if it had been attacked and eaten into by the petro-
liferous portion of the rock. Botryoidal masses of calcite are
present close at hand, washed out of the clay series.
The suggestion is made that these botryoidal masses repre-
sent calcite that has been dissolved out of the sandstone and
has crystallized out in the softer clay, thus leaving room for
the oil to impregnate the sandstone beds. In the zone of
weathering, carbon dioxide and water might be present in
sufficient quantity to attack a calcareous cement, but the action
must have taken place beneath the surface to allow the dis-
solved calcite to concentrate under concretionary action and
crystallize out.
What part the petroleum and its accompanying gases can
have taken in such an action it is difficult to determine ; with
the help of water they may have supplied the corrosive solution.
The point to be noted is that these phenomena have only been
observed where oil-bearing strata are present. Further study
of such evidence may throw light upon the movements and
storage of oil, and especially upon the effect of oil and water
in combination upon limestones, and may help to explain the
selection of beds to form oil reservoirs, even when they are
surrounded with almost impervious strata.
There are many minor points with regard to the under-
ground storage of petroleum which might be cited, but all
depend upon the principles already laid down, the selection of
the most porous or potentially most porous stratum available.
The migration through practically impervious beds must be
very gradual, but, given sufficient pressure, it is sure, though
it is probably only the lighter constituents of the mixed hydro-
carbons that are able to migrate for any considerable distance.
CHAPTER IV
LATERAL VARIATION
HAVING now considered most of the more important theoretical
questions concerned with the formation, migration and storage
of petroleum, let us turn to the more practical matters of how
oilfields are to be found, and how we can make as sure of them
as possible. In the next five chapters facts as discovered and
studied in the field will be considered, and theory as far as
possible eschewed, while methods of approaching the various
problems which have been found of value by the writer will be
discussed.
The geologist whose task it is to prospect a new country, or
a new area in a well-known country, for petroleum, will do well
to prepare himself by the collection of as many previously
known facts as he can find bearing upon the particular area,
and by the deliberate abstention from reading any opinions,
generalizations, or theoretical matter that have been published^
about it. By this the intention is not to cast aspersions upon
any work done previously by explorers, geological surveyors, or
travellers with a taste for science, but simply to enable the
" field-student " to start work with a perfectly open mind. The
line between opinion and fact must be drawn rigidly. There are
very few countries nowadays which are not, at least, partially
known geologically, and geological surveys, even if only of a
pioneer type, have done much excellent and sometimes even
detailed work in many parts of the world ; but the generaliza-
tions into which the pioneer geologist is inevitably tempted are
dangerous things, and lest they should impress, oppress, or
antagonize his mind, the field-student will do well to know
nothing about them. Ready-made generalizations fit the facts
no better than ready-made coats fit the body ; they are the bane
of original work, and unless the observer can improve upon
56
LATERAL VARIATION 57
what has been done before, and can see a little deeper into the
geological puzzles that await him than has been done by
previous workers, he is unworthy of his task.
To get at recorded facts, however, without absorbing opinions
is a matter of difficulty, but for this reason the writer would
emphasize all the more the necessity of an open mind. After
field-work has been done, new facts collected and correlated,
new areas mapped, then comes the time for reading, for testing
theories and opinions in the light of new discoveries, and one's
own theories in the light of how such or similar facts appeared
to trend in the minds of others.
Let a small scale geographical map of the country be
procured (if there be such a thing as a geological map it will
also be necessary), and let the prospector sit down before them
and study them, noting roughly on each such essential facts as
the ascertained or reported occurrences of surface indications of
oil. If only an unknown or unprospected district of a country
is to be examined, a map of the said district will not suffice ;
a map of the whole country, perhaps with portions of neigh-
bouring countries, is essential. The " field-student," as the
writer prefers to call one who reads the rock rather than the
printed page, who travels through countries rather than
reference libraries, is now in search of a few general ideas.
What if they prove wrong ? It is no matter ; they will be
^> tested in the field.
The orientations and extents of the principal mountain
granges, the courses of the main rivers, and the character and
configuration of the coast-line, if any, are naturally the first
points to be noted. The first of these will probably give a very
clear indication of the directions of the principal earth-move-
ments to which the area has been subjected, or at least will
show that one of two directions at 180 degrees is the main
direction of the principal or latest movement.
The courses of the rivers can as a rule be divided into
" consequent " and " subsequent " portions, and will thus in
connection with the mountain chains afford considerable assist-
ance in determining roughly the main strike-lines of the
country.
A study of the coast-line should indicate what parts are
rocky and what parts flat and low-lying, and the presence of
any delta of considerable size will be detected at once.
58 OIL-FINDING
If the oilfields to be searched for or examined are in
Tertiary strata, the methods of arriving at general ideas are
simple, as it is only the latest earth-movements that have to be
considered. If series older than the Tertiaries are to be
examined the enquiry becomes more complicated, and ifc may
not be possible to arrive at any general ideas of importance by
a preliminary study of the map, unless some geological data
are available. Most of the world's great oilfields, however, are
in Tertiary strata, and of oilfields yet to be discovered in such
countries as Galicia, Roumania, Eussia, Egypt, Turkey, Persia,
Baluchistan, India, China, Venezuela, Columbia, Brazil,
Argentina, and Mexico, we may safely assume that very few,
if any, will be in rocks older than the Cretaceous formation ; so
for the present let us consider that a Tertiary Series is to be
prospected.
Some general ideas as to the probable main structural lines
of the country having been obtained from the map, and the
approximate positions of known and reported indications of
oil noted on it, the prospector must ask himself why the oil
is found in such localities and how it got there. These queries
may take long to answer, or to obtain any light upon; when
they have been answered, the prospector will be in a position
to determine where else petroleum is likely to be found. The
reason for considering such queries and attempting to find
answers to them is that the general question of the occurrence
of petroleum should not be lost sight of when practical field
work is begun. To search for favourable structures in areas
which are apparently outside the belt of country in which it
is possible to find oil in paying quantity is not only a waste
of time from the practical point of view, but, if experimental
wells be drilled as the result of the prospecting work, other
instances will be added to the long list of failures which have
made the general public look upon oilfield work as on much
the same level, in regard to risk, as gold mining.
The prospector is now ready to familiarize himself with the
lithological characters of the rock with which he has to deal.
For this purpose several lengthy traverses across the main
strike-lines of the country are necessary, and also, if possible,
one or more roughly along the strike. The object is not only
to study the series as a whole, but to determine, if possible
the direction or directions of lateral variation.
LATERAL VARIATION 59
This is primarily a more important matter than the study
of structure, and accordingly it is considered first. In the
author's experience are only too many instances of the follow-
ing of structure in oil development work, while the lateral
variation in the strata was neglected or lost sight of.
In many cases the working out of the directions of variation
in the field maybe a laborious task, necessitating the determina-
tion of the stratigraphical relations of different groups, but in
some cases a clue may be furnished at once from the preliminary
study of the map. There may be a great river in the country
with a well-marked delta, and the evidence may point to this
river having been represented in Tertiary times, while the
ascertained main directions of earth-movement, or earth-waves,
may indicate in what direction, laterally or otherwise, the
course of the river has probably been changed between Tertiary
times and the present day. In Persia, Burma, and Baluchistan,
this method of approaching the subject proved of great value.
Deltaic Conditions. — If deltaic or estuarine conditions on a
large scale can be proved to have occurred during the Tertiary
Series in question, rapid and remarkable variations both along
and across the main strike-lines are almost certain to be
revealed. The field-student must look for constantly alter-
nating types of deposit, e.g. shales or clays alternating with
sands. Beds of undoubtedly marine origin, fine clays, marls
and true limestones, must be differentiated from literal or
deltaic deposits. In every case when examining a bed the
geologist must consider under what conditions it has been
formed. The only satisfactory method of arriving at a con-
clusion on such a point is to consider under what conditions
he has seen similar beds being formed at the present day, and
failing such direct evidence from his own experience, he must
consider under what possible conditions could such a bed be
formed. In such an enquiry there is no piece of evidence that
is too insignificant to note down. It may be that long afterwards
much importance is found to attach to items of information
jotted down in note books, or better still on the field maps,
items which at the time seemed to be entirely insignificant
details. The presence of gypsum or selenite in the clays, of
glauconite in the sands or argillaceous sands, and of remains
of terrestrial vegetation in any bed, must always be noted.
These all point to estuarine or deltaic conditions.
60 OIL-FINDING
In a general way the main directions of lateral variation
may be indicated from the very start by the records of former
observers, e.g. the presence of thick clays or limestones in one
district, and of coals or lignites in the same series in another,
at once suggests that some variation may be expected in such
and such a direction, even though the horizons of the particular
deposits have not been ascertained.
In the deltas of great rivers, channels are continually
changing their courses, so that sand-bearing currents trespass
upon mud-flats, and the coarser and more arenaceous detritus
thus alternates with the finer and more argillaceous. Sand-
bars are continually being formed between sea and delta,
cutting off lagoons or salt swamps. These sand-bars also are
subject to sudden modifications through the action of tides and
currents : they may be extended and increased, pushed forward
or thrown back, cut off to form shoals or completely swept
away. And with them the fate of the lagoons which they
protect is inseparably bound up. They may be filled up with
detritus to form solid ground, or may pass through a stage of
mangrove swamp to become a forest-lagoon, a forest growing at
or even under sea-level, where terrestrial vegetation flourishes,
dies and accumulates in masses which, under favourable con-
ditions, will in time be represented by coal or lignite seams or
petroleum. Any slight set-back in deposition, any temporary
gain of subsidence against sedimentation, and the lagoon will
be invaded by the sea, the vegetation killed, though perhaps not
washed away, and marine sediments may be deposited above
the remains of forest or swamp growth.
Speaking generally, however, a delta is always advancing
in one direction, in spite of the many deflections of the main
river-channels. A delta in fact means a victory of sedimenta-
tion over subsidence, and in any area where deltaic conditions
can be proved to have existed for a long period, littoral sediment
will be found to have advanced over more purely marine or
pelagic deposits. Set-backs no doubt frequently occur, owing
to periods of more rapid subsidence, but a delta stands for
continuous deposition, and till checked by a movement of
upheaval which is sufficient to enable the river to denude its
own deposits, or by the encounter with powerful ocean currents,
it must continue to advance.
In such circumstances it is obvious that rapid lateral
LATERAL VARIATION 61
variation must occur somewhere at every horizon. In some
cases the variation is very remarkable. The Tertiary Series in
Trinidad, formed as it is largely of fluviatile, deltaic, and
estuarine deposits at the mouth of a great river, which is now
represented by the Orinoco, affords some very striking instances.
The island is situated on the margin of a continent with the
deep Atlantic basin not far to the eastward, and the strata
in the Tertiary Series represent a continual struggle between
pelagic and deltaic strata, with the latter gradually becoming
predominant, and variation on the same horizon is remarkably
well shown. Thus near the Cunapo lignite field it is possible
to pass on the same line of strike from a lignite seam through
conglomerates and sands representing a littoral deposit into
muds, fine clays, and finally a marine limestone within a
distance of three or four miles. The lateral variation in that
island is very complicated, and has not been fully worked out
on all horizons as yet, but it seems to have been lost sight of
by many of the energetic oil-prospectors who have visited the
Colony.
In examining a deltaic formation, then, variation in
almost every direction may be observed locally, but the
algebraic sum of all variations, supposing it were possible to
measure these effects, would point to some general direction.
The concrete bits of evidence to be looked for are the
splitting up and thinning out of sandstone beds, the decrease
in coarseness of arenaceous sediment, the passage of sandstones
into thin calcareous sandstones among argillaceous rocks or
finely laminated alternations of sand and clay, the oncoming of
finely laminated clays without gypsum, and the directions in
which they thicken. Similarly the development and thickening
of beds of calcareous marl, whether foraminiferal or not, and
the first signs of true limestone bands must be noted. A shell-
bank, formed of a mass of broken shells on a shore line, must
not be considered as a limestone, even though it may be com-
posed almost entirely of carbonate of lime ; it has been formed
in the same manner as a littoral sand. Again, the thinning
and splitting up of lignite seams among banks of sand and
conglomerate, which were the bars between sea and lagoon,
the passage of such seams into carbonaceous sands or clays,
and the passage of shales into underclays and leaf beds, are
of great importance. All these phenomena, if observed carefully,
62 OIL-FINDING
will give definite information as to which side the land lay and
which side was open sea.
All evidence of shallow water conditions or sub-aerial
conditions such as false-bedding, ripple mark, sun-cracks, rain-
pittings on fine sands and clays, and in some countries deposits
of lateritic type, which were weathered and oxidized at the
time of formation, and represent what were at one time land-
surfaces, are of value.
The directions of currents can frequently be made out
from the arrangement of the longer axes of the pebbles in a
conglomerate, and especially in clay-gall beds, and what have
been called " clay conglomerates," which consist of pebbles of
more or less soft argillaceous beds in a sandy matrix. This
type of deposit is caused by a sand-bearing current impinging
upon a partially consolidated clay or mud deposit and breaking
up the bed, rolling the fragments into pebbles, and often
bending the pebbles so formed. They pass, by the gradual
decrease in the size of the argillaceous fragments, into sandy
clays. Beautiful examples of this type of deposit can be
seen on the western coast of Trinidad, and may be photo-
graphed in cliff sections, where the actual initial bending
up and breaking of the argillaceous bed is sometimes observed,
the current action having been checked exactly at this stage.
In Burma also, and in Persia, where the detrital limestones
have thinned out and become muddy and sandy, bands formed
in this manner are to be seen.
Finally, fossil evidence must be studied in connection with
these variations, but the fossils must not be taken from the
mixed faunas formed in littoral beds, where specimens from
littoral, laminarian, and pelagic zones are washed about on
the beach together, but from the actual deposits in which
or on which the organisms lived. Oyster beds, for instance,
may be noted as important ; foraminiferal beds, thick clays
containing lamellibranchs with joined and closed valves and
gasteropods in perfect preservation, and assemblages of fresh
and brackish water forms are all of help in determining
directions of variation.
In well-exposed sections on river banks, sea cliffs, road
or railway cuttings, and, if the ground be not too much
obscured by vegetation, in any hilly ground, it is possible
to study all these phenomena and to derive from each some
LATERAL VARIATION 63
link in the chain of evidence as to the side on which sea
and shore respectively lay, while the deposits were being
formed.
When this study is extended over wide areas and over
many horizons in a thick series, the course of a delta can be
made out with considerable accuracy at each successive epoch,
and it can be shown to have pushed forward its littoral sedi-
ment, now rapidly, now slowly, with recurring intervals of
retreat, and with perhaps slightly diverging directions at
different times, but over all with a steady inevitable advance
over the more characteristic marine sediments with which it
was contemporaneous.
The Pegu Series of Burma, ranging from the Eocene far up
into the Miocene according to our subdivisions of Tertiary
time, furnishes perhaps the most conclusive evidence of the
advance of a delta that has been worked out in any detail.
All the phenomena of deposition mentioned above can be
studied in this series, but for the most part the thinning out
and splitting up of sandbeds, and the simultaneous thickening
of clays, are sufficient to make the directions of variation
quite clear, so that we are now enabled to elucidate the history
of the series in almost all parts, and to give an idea of the
conditions under which each particular bed was formed. The
boring journals of oil wells have been of the very greatest
assistance in establishing the history of the Pegu Series point
by point.
A great river flowing from the northward entered a land-
locked gulf, and hugging the western shore, gradually filled
it up by its advance. Much of the axis of the Arakan Yomas
was already land when the Pegu Series began to be deposited,
and along the western shore thus formed great littoral sand-
stones with much evidence of terrestrial vegetation were
deposited. On its eastern side the deltaic deposits were inter-
calated with truly marine beds. The advance of the deltaic
deposits was not steady, but subject to many checks and
retreats. During some of the checks vast accumulations of
vegetable matter were formed in the swamps and lagoons near
the river-mouth, to be afterwards buried under marine deposits
of the invading sea or covered by coarser estuarine detritus
as the delta was pushed forward. Thus in Lower Burma the
oldest strata of the Pegu Series are entirely or almost entirely
64 OIL-FINDING
marine, while deltaic and even terrestrial conditions existed
simultaneously in parts of Upper Burma. Earth-movement
was in evidence, but not very active.
When the delta passed beyond the shelter of the western
coast its course was to some extent deflected by ocean currents,
but it continued to advance over the marine sediment. Now,
after many changes of level, and the deposit of an overlying
flu via tile series (which is usually unconformable to the Pegu
Series, but also has a marine phase that cannot but be conform-
able somewhere to the Pegu Series), the Irrawaddy has fallen
heir to the former great river ; and though it has a somewhat
different course the general direction is the same, and we can
still study the advance of a delta at its mouth.
In other countries the advance or retreat of deltaic fans of
detritus may also be proved, but few cases are so simple as
that of Burma. In Trinidad, for instance, earth-movement on
a fairly large scale supervened during the deposition of the
Tertiary Series and caused certain complications, so that the
upper strata lie in a violent unconformity across the denuded
strata of Middle Tertiaries. Yet the main directions of lateral
variation can be proved with some degree of accuracy. During
the deposition of the earliest Tertiary strata, sedimentation
was advancing from the south-east, while marine conditions
persisted for a longer period in the south-west. Towards
the middle of the Tertiary period sediment was poured in from
the south and west, and the arenaceous detritus is intercalated
with and passes into pelagic strata to the north and north-east.
Then followed a period of retreat when the advancing arms of
the delta barely held their own, and fine clays and forarniniferal
marls were deposited above strata of deltaic origin. Finally,
sedimentation advanced again, and arenaceous strata were
deposited by many currents flowing in various directions, east
and west and north. The presence of islands of older strata,
and the inception of a folding movement acting in a northerly
direction, introduced many complications, but the main branches
of the delta can be followed out, and seams of lignite and bands
of oil-bearing rock at various horizons mark approximately the
localities where accumulations of vegetable matter in forest
lagoons or swamps were formed.
The same principles may be applied to the elucidation of
the history of the Tertiary Series in many other countries, but
LATERAL VARIATION 65
it would weary the reader to enter into elaborate details of the
evidence from one country after another that has served to
confirm the theory and to associate oilfields with deltaic
conditions.
The point of all this insistence upon the importance of
studying lateral variation and determining the boundaries of
a deltaic formation is simply that the probable petroliferous
belt may be recognized. If oil is to be drilled for, it is as well
to look for it as near as possible to areas where the conditions
for its formation were favourable. On the one hand may be
littoral and terrestrial beds where the carbonaceous phase is
in evidence, on the other marine beds beyond the confines of
the delta. Somewhere between we may hope for an area where
the necessary alternations of arenaceous and argillaceous
sediment are present, an area not too far from localities where
accumulations of vegetable matter have probably been formed,
buried, and sealed up. In that area we must seek for favour-
able structures to concentrate and retain the petroleum.
Much excellent work of competent geological surveyors,
much arduous toil in opening up new districts and transporting
plant to them, much fruitless expenditure of money and time
could have been saved, had the facts concerning lateral varia-
tion been carefully studied and mastered.
It is, of course, a commonplace of geology that lateral
variation occurs in rocks of all ages, but unfortunately in
Britain, the birthplace of stratigraphy, the variations in most
formations and series, with a few notable exceptions, are not
very great, and much correlation of strata, it is to be feared,
is still attempted chiefly on lithological grounds. Where we
have evidence of continuous deposition on a large scale, and
deltaic and estuarine conditions, as we have in the Carboniferous
Formation, it has taken decades of field-work and controversy,
and volumes of scientific papers written and discussed before
it has been possible to arrive at a conclusive general idea of
the conditions and variations. Even at the present day we
find a classification based upon subdivisions established in
some districts in England, including the well-known Millstone
Grit, forced upon Scotland where such a classification is neither
natural nor of practical benefit; and still " pakeontological
breaks" are inserted in a continuous series in order that a
universal general arrangement may be adhered to. Similarly
F
66 OIL-FINDING
we have seen our local subdivisions such as Eocene, Oligocene,
Miocene, and Pliocene forced upon other countries where their
only significance is chronological, and where no natural group-
ings can be made to coincide with them. In our turn we
have adopted Continental subdivisions, e.g. in the Cretaceous
Formation, which are at the least very doubtfully applicable.
But if lateral variations during continuous deposition can
be proved to be common and distinct among the primary and
secondary formations in the Tertiaries almost all over the
world, they become even more frequent and impressive, because
it is those Tertiary strata which have emerged comparatively
recently from beneath sea-level that we know; the great
uniform Tertiary deposits, which perhaps future geologists will
examine, are still beneath the waves. That is to say, we know
only the margins of the Tertiary formations, and it is precisely
along land-margins and on the fringes of continental areas
that lateral variations must naturally be greatest.
"We must take variation, then, as the rule and not the
exception when studying Tertiary strata, and must not attempt
the correlation of distant areas, as the author has often seen
done, by similarity of lithological characters or the presence of
some particular mineral or minerals. Such evidence only
means similarity in the conditions of deposition, a similarity
which during progressive sedimentation must migrate from one
area to another. Thus oil-forming and oil-bearing conditions
may be transferred from the lower beds of a series to the upper
beds as we proceed from one province to another.
CHAPTER V
GEOLOGICAL STRUCTURE
IT will be noticed that what is usually considered the most
important matter in oilfield work, the study of geological
structure, is given a secondary place. This has been done, not
because its importance is not fully recognized, but because the.
study of lateral variation comes first naturally, and may render
unnecessary a great deal of detailed work in discovering and
mapping favourable structures. The field-student has now
advanced sufficiently in his knowledge of the country which he
is examining to be able to predict in what districts, and
possibly also at what general geological horizons, conditions
are favourable to the formation of petroleum. His next task
is to discover in the indicated districts suitable structures to
contain and preserve the petroleum from inspissation, and to
ensure sufficient concentration to make paying productions
probable.
Earth-movements. — The elucidation of geological structure
naturally depends on a study of the earth-movements that have
been experienced. Here, again, an examination of the general
map of the country is essential ; it is advisable to get a broad
view of such evidence of folding, faulting, and unconforma-
bilities as has been obtained before details of structure are
attacked.
In the preliminary traverses made to gain an insight into
the lateral variation, the geologist has doubtless obtained some
evidence of folding and the direction of folding movements.
In many cases only one earth-movement will require to be
considered; in others two, or even more, of different ages,
directions and degrees of severity may have to be distinguished
and delineated. Earth-movements are instances of relative
movement, but as a rule it will be found simpler to consider
them as movements in a definite direction towards some central
67
68 OIL-FINDING
axis of folding, the force being applied tangentially to the
earth's crust. There is no structure produced by flexuring or
faulting that cannot be explained by the application of this
simple principle.
The movement to be considered, then, resolves itself into
a horizontal push, and in the majority of cases the direction is
from the seaward towards the mountain ranges. Again, the
oldest strata exposed will be found as a rule in the heart of any
axis of folding that may be present. This gives another method
for determining on prima facie evidence the direction of move-
ment.
Where flexuring attains to great dimensions, and a series
of well-marked parallel folds is produced, the steep sides of
asymmetrical folds will be found almost invariably on the
side from which the movement took place, and vertical or even
inverted limbs of flexures are not uncommon in such circum-
stances, even among comparatively young Tertiary . strata.
This is all in accordance with the development of a geanticline,
and the production of a facher or fan structure, in which the
axial plane of the central fold is vertical or nearly so, and the
axial planes of the sharp flanking folds dip towards the central
axis. As one. recedes from the central axis of folding, the
flexures gradually become less sharp and more symmetrical,
till finally they may be represented by small gentle undulations
or elevations which affect the dip of the strata so slightly that
the pocket clinometer may not be sufficiently accurate and
sensitive to prove a general dip in any one direction. In both
Persia and Burma there are excellent examples of a series of
flexures rapidly decreasing in sharpness as we recede from the
central axis of folding.
Thus when a wide area is to be examined there is usually
little difficulty in determining the direction of movement, and
after a traverse across the main strike-lines of the country it
should be possible to predict where gentle folds should be in
evidence, where sharp or asymmetrical flexures, and where
inversions of the steeper limbs of flexures may be expected.
The age and duration of the earth-movements is another
matter of great importance. In some cases it will be found that
movement has proceeded fairly steadily during the deposition
of a Tertiary series, causing older strata to be elevated on the
crests of flexures, brought into the reach of denuding forces and
GEOLOGICAL STRUCTURE 69
actually denuded, while continuous deposition was proceeding
in the synclines. The results produced are violent uncon-
formities along certain lines, with the unconformability dying
out laterally, while the older strata may be seen in localities
sharply folded and overlaid conformably by successive younger
strata in which the dips decrease steadily upwards, the upper-
most beds perhaps being practically horizontal. The best
instance of this that has come under the writer's observation
is in Persia (cp. Plate VII), where the movement has been in
operation since early Miocene times and is probably still
continuing.
In other cases the movement may have been long-continued,
but not continuous, so that at several distinct epochs, separated
by intervals of quiescence, it has been rapid. The results will
be the production of local unconformabilities at different stages,
but these unconformabilities may die out laterally within a
comparatively short distance, and must not be treated as if
they were universal. Great lateral variations in the strata
will be caused under such conditions. Sind and Baluchistan
afford a good instance of this. In the records of the Geological
Survey of India dealing with this province it will be seen that
a great number of types of sediment are represented, and very
frequently they are separated by unconformabilities. The
unconformabilities in this case are almost entirely of local
importance only ; there is great lateral variation, but there has
been little denudation of previously formed beds throughout
the series from early Eocene up to perhaps middle Miocene.
In other cases definite periods of folding movement may be
made out, and the relative ages of each can be determined by
the effects upon series of different ages. This is the case in
Burma, where one movement has been detected that affects the
Pegu Series and earlier strata, while another and much greater
movement in a different direction affects not only the Pegu
Series but the younger Irrawaddy Series lying unconformably
upon it.
In studying earth-movement it must be remembered that
faults are part of the movement just as much as flexures. A
fault is merely a special case of folding, where the elasticity
of the strata or the amount of "load" is not sufficient to
prevent dislocation. Theoretically a fault may always be
replaced by a sharp monoclinal bend, and the passage from one
;0 OIL-FINDING
into the other may often be seen. It has too often been the
custom to think of a fault as something quite apart, and to
map a fault — to use a metaphor from whist — as one would
play a trump, not following suit. The author has known
faults to be recklessly strewn about a geological map in this
manner, when the presence of one could not be justified without
the mapping of two or three more which were entirely theo-
retical, for which no evidence could be obtained, and of which
the amounts and directions of throw were purely conjectural.
From such methods it soon arrives that when any structure
has not been elucidated properly, the remark will be made
"there must be a fault," and the puzzle is deemed to be
explained. A fault would thus become a sort of make-shiffc to
get faulty observers out of trouble. But the effect is just the
opposite ; such methods soon affect their own cure by involving
the geological surveyor in a net-work of physical impossibilities,
from which the only escape is to begin the mapping all over
again.
Faults are really very simple matters, and they obey
physical laws just as folds do. They must, therefore, only be
mapped when justified on physical grounds, and no fault must
be recorded of which at least the direction of throw, if not
some estimate of the actual amount of throw, can be given.
Structures Favourable to Concentration of Petroleum. — With
these preliminary remarks we may pass to a consideration of
the structures most favourable to the underground concentration
of petroleum. It is usual in books upon the subject to give a
list of the various structures which have been tested and
proved productive, and a great number of different classes of
structure can be described. But any one can be assigned to
one of a few main types. Anticlinal structure and petroleum
are associated in the minds of all who have studied or worked
in oilfields, and though petroleum can be proved to occur in
almost every known structure, in the vast majority of cases
some form of anticline is present in a successful field.
Dome Structure. — It will be admitted generally that the
most favourable structure of all is a dome or quaquaversal,
with gentle dips near the summit and steeper dips upon the
flanks, which again pass gradually and steadily into a position
of horizontality, so that a large area can be included as properly
GEOLOGICAL STRUCTURE 71
belonging to the dome. This is merely a special case of
anticlinal structure, an anticline with pitches of the axis away
from a central point.
A broad round dome is very rare in nature, as it almost
necessarily requires more than one earth-movement for its
formation. Elongated domes, however, are fairly common and
there are few anticlines that have not in one or more localities
some trace of dome-structure. Spindle-Top is probably an
isolated dome of breadth approximating closely to its length,
though the evidence does not seem ever to have been recorded
very clearly. The famous Yenangyoung field in Burma is
an elongated dome of great extent, nearly symmetrical, and
only slightly affected by purely local faults. It is isolated
from any other flexure by miles of approximately horizontal
strata, and thus drains a large area, while steep dips occur
on either flank, ensuring a high concentration of the petroleum.
About two square miles of available drilling area is provided
on its crest. It is obvious that under such conditions hydro-
static pressure of water in the strata is enabled to concentrate
the petroleum from all sides towards the summit. In such
ideal structures, whatever be the quality of the oil, and however
small the porosity of the oil-bearing strata, such a concentration
is bound to take place and large productions may be expected
whenever the oilsands attain to a reasonable thickness.
Symmetrical Anticlines. — Next in importance comes the
simple symmetrical anticline (cp. Plate VIII), either without
pitches of the axial line, or with pitches too low to have
affected the structure favourably or unfavourably. Many of
the Eastern fields in the United States have structures of this
nature, the anticlines, though extensive, being often so low
and flat as to be only distinguishable by very careful levelling
or by evidence from actual bores. It is obvious that the
greater the extent of the flexure, the greater should be the
concentration of oil towards the crest, given sufficient hydro-
static pressure. The effects of pitches and gentle dips will
depend to a great extent on the nature of the oil and the
porosity of the oil-bearing strata ; the greater the specific gravity
of the oil and the smaller the porosity, the slower and less
complete will be the migratory movement. Thus structures
that have proved remarkably favourable for the production
of a light oil of paraffin base, may not cause any great
72 OIL-FINDING
concentration of a heavier grade of petroleum. In the United
States the fields of New York, Ohio, Pennsylvania and Virginia,
where as a rule the oil is light and mobile, show many instances
of very flat structures, anticlines with slopes of twelve feet in
a mile, for instance. It was in these fields that drilling for
oil was first attempted and learnt, and they were taken as the
type of what oilfields should be. This idea still survives to
some extent in spite of the discovery of so many great fields
with totally different structures. The advantages of these
eastern fields in the States are many ; horizontal or low dips
make the easiest drilling, and the strata being palseozoic are
mostly fairly hard and not very liable to " caving," so that
drillers trained in these fields were acquainted with few of the
difficulties attendant on drilling in soft Tertiary strata. When
oil began to be discovered in other provinces under entirely
different conditions, many a field that has since proved very
profitable was condemned at first because it did not conform
to the structural peculiarities deemed essential in the fields
of the Eastern States, and many a practical operative, with only
experience of the Eastern fields, proved a failure when con-
fronted with the task of drilling through soft and steeply
dipping Tertiary strata.
Asymmetrical Anticlines. — Another form of structure that
has provided many excellent fields is the asymmetrical anti-
cline. This is an anticline with one flank gently and the other
steeply inclined ; the latter in some cases may be vertical or
even inverted. Such flexures usually occur nearer to the
central axis of folding than symmetrical flexures. The " terrace
structure," well known and much sought after in the eastern
fields of the United States, may be regarded as a special case of
this form of structure, an anticline so flat and gentle that the
gently dipping flank is for all practical purposes horizontal.
In sharp asymmetrical anticlines such as those at Kasr-i-Cherin
in Persia, and Yenankyat and Singu in Burma, it is evident
that drilling on the actual line of crest is useless ; the drill
soon enters steeply dipping beds where great difficulties may be
encountered in the drilling, while there may be no possibility
of penetrating to a sufficiently low horizon. Mr. G. B. Reynolds
pointed this out in the Persian fields, and Mr. Pascoe in the
Eecords of the Geological Survey of India has since explained
the effect of such a structure in the case of the Yenankyat field
GEOLOGICAL STRUCTURE 73
in Burma. This point will be referred to later in the chapter
on " Location of Wells."
Compound Anticlines, — Compound anticlines may next be
considered. These are only observed where the flexuring move-
ment has been severe, and has produced a series of sharp folds,
possibly with very steep flanks. The most striking instance
that has come under the writer's observation is at Maidan-i-
Naphtun, in Persia, where a group of no less than seven sharp
local flexures is included in an area approximately one mile
in breadth. Some of the flexures are so sharp that when a band
of hard limestone is exposed on the crest it is possible to sit
astride on the anticline. Highly inclined strata are the rule
throughout this field, and vertical or inverted limbs of folds are
common. These seven flexures converge towards and pass into
one broader fold, which, being formed of strata less amenable to
distortion, is somewhat gentler, and which really gives the key
to the structure of the neighbourhood. The whole area is
broadly anticlinal, and the sharp folds are merely puckers upon
a well-defined flexure on a larger scale. Every well drilled in
the area, whether upon one of the minor anticlines or in one
of the synclines, has struck oil in paying quantity, but the
surface indications of oil are nearly all upon or close to the
crests of the minor flexures. These minor folds, though very
striking and impressive, can therefore be disregarded and the
compound anticline considered as a whole.
Synclines. — Mr. W. T. Griswold has pointed out that under
certain conditions synclines may be exploited for oil success-
fully. The necessary conditions are that the strata are not
waterlogged, and are so covered or sealed that water cannot
enter the oil-bearing bands at outcrop. In such circumstances
any petroleum in a porous rock will tend to collect at the
bottom of the syncline under the force of gravity. It is very
doubtful whether many such cases exist, though, in rainless
regions or where the bulk of the strata are practically im-
pervious to water, such conditions are possible.
With an oil of approximately the same specific gravity as
water, displacement by the water might be very slow and never
quite complete : so a certain quantity of the petroleum might
remain in synclines, especially if deposits of asphalt were
formed upon the outcrops by inspissation of the exuding
74 OIL-FINDING
petroleum and the downward percolation of water checked if not
entirely prevented. Thus, although with a light oil it may
very seldom be worth while to drill in a syncline, with heavy
asphaltic oils and in regions where rainfall is very small, shallow
synclines might be worthy of being tested, and might prove
highly productive. Even in Trinidad where the rainfall is high
there is some evidence in favour of making a test of one of the
shallow synclines, where extensive asphalt deposits cover most
of the outcrops of oil-bearing sand. Where oils are very heavy
and sluggish, a certain proportion of water in the oilrocks is
rather a benefit than otherwise, assisting the flow of oil.
Monoclines. — Finally, oil may be obtained from monoclines,
often in great quantity. In such cases the more gentle the
dip, the better, but even quite steeply inclined strata may yield
good productions. Many of the great oilwells in Kussia are
drilled into strata which crop out at the surface at no great
distance : the new and much boomed field of Maikop is shown
by the published geological maps to be in a outcropping series.
In Peru, Trinidad, and some parts of California and Mexico,
good productions have been obtained from beds that crop out
in monoclines. It is to be noted that in these cases the oil is
asphaltic and of fairly high specific gravity, the latter quality
being due, partially at least, to inspissation.
In Burma, where a light oil of paraffin base is the character-
istic petroleum, no adequate production has ever been obtained
by drilling in a monocline to strike an outcropping oilsand.
Native hand-dug wells have occasionally been worked at a
profit for a short time in such structures, and many trial bores
have been made at great expense, but all have been abandoned
finally as unprofitable propositions. Except where the strata
are for the most part impervious, and there is a probability of
striking some isolated lenticular bed of oilrock, there is little
hope of obtaining paying productions of light mobile oil by
drilling in a monocline where all the horizons crop out. Thus,
the class of oil to be obtained must be considered in relation to
the structure; conditions favourable for an asphaltic oil may
be quite unfavourable for a light paraffin oil.
It must not be forgotten in dealing with a monocline that
it is really part of a great curve, on the flank of either some
great anticline or syncline. Dips do not continue far at the same
angle when traced downwards, as the geologist will discover
GEOLOGICAL STRUCTURE 75
at once in plotting sections. Any sudden change in angle
of dip may have great effect on production. Thus, though a
well may begin in strata dipping at 45 degrees or more, by
the time the oil-bearing rocks are reached the dip may have
decreased to 20 degrees or less, or may have increased and even
become vertical. Each case must be worked out from the
geological map of the area, for it is generally quite absurd to
calculate on a dip remaining constant for any considerable
distance. Before any wells had been commenced in an area now
being exploited by a company in Trinidad, the writer (then
Government Geologist in that colony) worked out the vertical
and lateral changes of dip, where direct evidence of the inclina-
tion of the beds was very scanty and often unreliable, and
furnished those responsible for the exploitation of the ground
with particulars of the depth to the observed oil-bearing bands
in different places and the angles of dip at which each would
be struck, particulars which in the end were proved to come
within a very small fractional error of the actual results
obtained. In this case projection of the observed dips from
the nearest reliable section would have given an entirely
erroneous result, and would have made the area appear very
unfavourable for development work. Every bit of evidence
bearing upon change of dip must therefore be noted, and when
outcrops can be traced, even though no actual observation of
dip can be made, it is often possible to estimate change of dip
by measuring the distances between two known outcrops. The
writer has often found this method of great service in obscure
ground.
Where the dip in a monocline suddenly becomes shallower,
or where a sudden change of strike occurs, especially where a
bend in the strike concave to the direction of dip in the
monocline is observed, there is nearly always some favourable
effect upon the concentration of petroleum. The oil nearly
always is found to have migrated towards such structures.
This can be readily understood in the case of a bay or bend in
the strike which is frequently accompanied by a lowering of the
dip ; it is in fact an abortive anticline, since a tilting back of
the monocline to a position of horizontality would make an
anticline or dome of such a structure. Instances of this may
be studied in Trinidad, perhaps the most remarkable being at
Pala Seco near the southern coast on the northern flank of the
76 OIL-FINDING
great southern anticline. Structures such as this may be due
to movements earlier than that responsible for the monocline,
and the concentration of petroleum may have taken place prior
to the great movement which caused the main flexures of the
country. However this may be, such structures, bays in the
strike concave to the direction of dip, always favour migration
and concentration of oil.
Every structure produced by folding can be classed under
one or other of the heads detailed above.
Faulting. — Faulted areas are not to be regarded as a special
class of structure, but faults may be of great importance struc-
turally, so that it is necessary to give some account of how they
occur in oilfields and what effects may be attributed to them.
The flank of any fold may be replaced by a fault, partially
or wholly ; in this case it is a strike dislocation having the
same effect as a very sharp unbroken fold would have, and
obviously due to the same movement that flexured the strata.
In many cases what is mapped as a fault at the surface may
become a sharp flexure at some distance beneath, where the
load must have been greater during the period of movement,
and consequently a higher coefficient of elasticity of the strata
can be postulated. This applies both to " normal " and
" reversed " faults, the former being in evidence on the flanks
of symmetrical or gentle flexures, while the latter are frequent
on the steeper side of asymmetrical folds especially where
vertical or inverted strata are observed.
In areas under insufficient load the cohesion of the strata
may be overcome under flexuring stresses, and faults may be
developed in many directions, all, however, having some relation
to the flexure that the stresses are tending to form. Indeed,
it is only under such conditions that what is called, rather
unfortunately, a " normal " fault can be produced at all. The
" normal fault " of the textbooks, a dislocation with a vertical
or nearly vertical displacement, the downthrow being in the
direction of hade, is by no means a common phenomenon in
nature ; it is only under simple conditions that such dislocations
are physically possible.
A dislocation must begin somewhere and must die out
somewhere, so it can be regarded as a sag or tilt, and strike
faults whether normal or reversed, whether thrusts, "slides,"
or the doubtfully possible " lags " of some authors, are direct
GEOLOGICAL STRUCTURE 77
and inevitable special phases or flexuring movements. Dip
faults on the other hand may be simple tilts or sags, or may
have a greater or less horizontal component. Many dip faults
which map as normal faults, and are considered as such, can
be proved to be largely horizontal displacements, thus approxi-
mating to the nature of " wrench faults." Every fault must be
considered in relation to the flexuring movement, and thus the
observation of a fault should be a help rather than a hindrance
to the elucidation of structure, since, when once the direction
of throw has been determined, it shows at a glance what
tendencies of movement were induced in the strata in that
particular locality by the stresses to which they were subjected.
A map may be complicated by faults, but the structure should
be explained by them rather than rendered more difficult to
understand. The geologist who, after describing the structure
of an area, concludes by saying that " there seems to be a good
deal of faulting," or who tries to safeguard his views of a
structure by saying " there may be a fault/' which, if present,
will put a different construction on the evidence, admits by so
doing that he has not mapped the area, and has only the
vaguest general idea of its geological structure.
Of course in strata of Palaeozoic or Mesozoic age faults
may be very numerous and of many different ages, but we are
dealing principally with Tertiary rocks where the flexuring
and faulting are usually simple, and the stresses that caused
them easily understood. Petroleum is very seldom, if ever,
found in highly faulted and contorted strata of Palaeozoic
age.
Since faults and folds are parts of the same earth-movement,
the effects of faults upon an oilfield need not necessarily be
prejudicial ; strike faults may indeed help to ensure a greater
concentration of the petroleum towards the crest of a flexure,
and dip faults in a series where there are many oil sands may
bring about communication between different sands, and so
have a notable local effect upon production. Where the bulk
of the strata are impervious, an oilsand which would otherwise
crop out at the surface may be cut off by a fault and sealed
beneath impervious beds (Fig. 2), and thus yield oil under much
higher pressure when pierced by the drill than if it cropped out
in the vicinity.
To illustrate the interdependence of folding and faulting,
78 OIL-FINDING
and at the same time the effects of two folding movements
in different directions, we may take the Yedwet inlier in the
Magwe district of Upper Burma. This was the first area
FIG. 2. — Fault sealing up an oil-rush. 1. Oilrock; 2. Impervious
strata.
examined by the writer in Burma, and it proved a very
fortunate one on account of the importance of the evidence
obtained.
The area consists of an inlier of the Pegu Series, surrounded
by, and in places capped by outliers of, the uncon form able
fluviatile Irrawaddy Series. The inlier has an oval outline,
and dips are gentle throughout, seldom rising to more than
20 degrees, and that only towards the margins. Presumably,
then, the structure was dome-like. Careful examination proved
that there was evidence of two flexuring movements, both very
gentle, one tending to produce flexures running E. 20 degrees N.
to W. 20 degrees S., of which two were recognized in the area,
and another tending to produce flexures running almost north
and south. The general form of the inlier is due to this latter
movement, which was easily identified with the main flexuriug
movement of Burma. The presumption was that the former
movement was an earlier one, which had not been recognized
previously in Burma.
The flexures are so gentle that a very simple case of the
dynamic conditions produced by one movement on the results
of an earlier one is presented. The strata had evidently not
been under great load at the time of the last movement, as
a number of small faults were detected in various parts of the
area. These faults are of the same age, they run into each
other, and no fault displaces another. They have therefore
GEOLOGICAL STRUCTURE
79
evidently been caused by the same movement. There are two
main directions for the faults, and though there are local
modifications and variations, the lines along which dislocation
has taken place are wonderfully constant throughout the area,
viz. E. 20 degrees N". to W. 20 degrees S. and roughly north and
south. That is to say, the systems of faults are parallel to the
strike-lines produced by the two movements.
The process which caused these faults can be expressed very
simply by a diagram (Fig. 3). Assuming that the movement
H
FIG. 3.— E F = crest of earlier flexure ; G H = crest of later
flexure ; h — faults showing down-throw.
producing the two flexures running E. 20 degrees N. to W. 20
degrees S. is the earlier, we have a force AB, impinging obliquely
upon a flexure EF. The force will be resolved into two com-
ponents AC and AD, one tending to increase the height of the
fold and the other tending to compress it and to raise a flexure
in the direction BD. It is a simple parallelogram of forces. Let
8o OIL-FINDING
us consider how different parts of the area will be affected. A
point upon the crest of one of the cross flexures will tend to
rise, especially where the effects of both movements coincide.
On the other hand a point in the syncline between the two
cross flexures will be affected differently according to its
position with regard to the second movement. Towards the
margins of the area a point in these synclines will tend to sink,
towards the centre of the area one component will tend to make
it rise and another to make it sink. The strata in such localities
will be under a peculiar condition of stress, and adjustment
will be arrived at by the developments of small faults. Thus
we get, so to speak, strike faults of both movements, though
produced simultaneously and the directions of throw will be as
shown in the diagram. All these faults were noted and mapped
before any theoretical ideas as to their origin were conceived.
Keplacing the faults by folds the action is quite easily intelligible,
and can be reproduced experimentally.
The evidence obtained from this area was applied to other
fields in Burma, and served to explain the presence of faults
in many localities where physical reasons for their origin had
not been ascertained. One of the first results was the proof of
the relative ages of the two movements ; that responsible for
the cross flexures was found not to affect the younger Irrawaddy
Series, while the other movement throws it into great folds.
The cross movement is therefore the earlier.
The formation of dome structures, which are common in
inliers of the Pegu Series, was also accounted for. It is almost
entirely due to elevation by the earlier movement which,
though always gentle, has had the effect of raising parts of the
series locally before the commencement of the great movement
which has produced the main strike-lines of the country.
Another interesting point is that petroleum has never been
obtained in paying quantity in any field that does not show
some traces of the earlier movement, even though these traces
are often almost obliterated by the much more powerful later
movement. It would seem that there has been a preliminary
concentration of the petroleum contents of the strata towards
the earlier flexures, which concentration has been greatly
increased afterwards by the later and greater flexuring.
It is, of course, only in simple cases that such conclusive
results can be obtained with certainty, but the Yedwet inlier
GEOLOGICAL STRUCTURE 81
will serve as an example of how the evidence from one area
may assist in elucidating the more complex structures of other
areas, and of the necessity for considering flexures and faults
together and not separately.
Unconformabilities. — The only other phenomena of import-
ance that must be considered when dealing with the geological
structure of a country are those associated with unconforma-
bilities. Unconf ormable junctions of different series, or of
different parts of the same series, may often be the cause
of considerable difficulty in the working out of the structure
of an oil-bearing territory. Among Palaeozoic or Mesozoic
rocks there may be no difficulty in recognizing an uncon-
formity, but in soft or lightly compacted Tertiary strata the
discordance may never be seen in actual section, while the
rocks both above and below may have very similar lithological
characters, and fossil evidence may be wanting or too scanty
and too little known to be conclusive. In such cases there
is a danger of an unconf or inability being unnoticed, with
disastrous results when estimates of thickness of strata and
depth to oil-bearing horizons are being calculated.
When proved, however, by careful mapping, an unconforrna-
bility may be of very great assistance to the field student in
giving evidence as to the age and nature of the earth-movements
in the country that is being examined. A case has just been
quoted from Burma, where the proof of the relative ages of
two movements depended on the evidence from strata of two
different series, separated by an unconf ormability. Had the
unconformability between the Pegu and Trrawaddy Series not
been ascertained, this valuable evidence would have been lost.
The discordance between these two series, moreover, is some-
times not easily recognized, the junction has several times been
described as a passage, and at one time the Geological Survey
of India were actually mapping one of the locally basal beds
of the Irrawaddy Series as the topmost bed of the Pegu Series.
The recognition, therefore, of an unconformity becomes a
matter of very great importance, and it must be distinguished
from a plane of merely local or contemporaneous erosion.
Local erosion is very frequent among rapidly accumulated
Tertiary strata, such as are characteristic of areas where deltaic
conditions occurred on a large scale. At the base of every
thick or coarse-grained arenaceous group, where it rests upon
82 OIL-FINDING
finer argillaceous sediment, there are nearly always some signs
of erosion of the underlying strata, and the divergence in strike
and -dip of the arenaceous group if current-bedded may be
considerable, so that in a small section the appearance of a well-
marked unconformity may be presented. On the other hand
a great unconformability, representing a gap in the geological
record, may appear in small sections to be a perfectly con-
formable sequence.
Careful mapping on a large scale will always make certain
of any unconformability of importance by disclosing the over-
lap of one series on the other, but where evidence is very
scanty, or when there is not sufficient time available for
detailed work, other methods must be relied on.
The presence of some mineral or minerals or fragments of
rock in one series and not in the other is a bit of evidence to
be noted at once, as it points to the strata having been formed
from the denudation of different rocks, so that if any such
sudden change in raineralogical composition is detected and
proved to hold good over any considerable distance and through
thick masses of strata, a prima facie case for the presence of
an unconformability has been made out.
Differences in the state of mineralization of the strata are
also to be noted, the bedding, lamination and jointing may be
of different characters in two apparently conformable series,
and finally, and most important of all, the lower series may
show evidence of small folding or faulting movements that do
not affect the upper series.
In the case of the unconformability between the Pegu and
Irrawaddy Series in Upper Burma, the point was established
beyond doubt by evidence obtained from an area far to the
northward of where the question had first arisen and become
of importance. The Irrawaddy Series was found some seventy
miles to the northward to contain evidence of volcanic action
several hundred feet above its base. This evidence included
outflows of lava, and formation of explosion-craters with beds of
ash and agglomerate. The ashes contained many blocks and
fragments of metamorphic rocks, abundance of acid lava bombs
and fragments including beautifully silicified rhyolites. The
strata of the Irrawaddy Series above the volcanic beds were
found to contain pebbles of metamorphic rocks and agate, and
occasionally much decomposed felspar and kaolin.
GEOLOGICAL STRUCTURE 83
The basal beds of the Irrawaddy Series in the Magwe
District to the southward, where the unconformability was in
question, contain well-rolled pebbles of metamorphic rock and
agate (from the silicified rhyolites), while kaolin was found in
some of the sands. None of these appear in the underlying
Pegu Series, and it was the occurrence of kaolin in a bed, then
considered to belong to the Pegu Series, that first turned the
attention of the writer to the possibility of there being an
unconformability. Examination of intervening, but discon-
tinuous, areas, gave confirmatory evidence, and it became clear
that the basal beds of the Irrawaddy Series, in the Magwe
District are post-volcanic, and that the pre-volcanic beds of
the Irrawaddy Series, some hundreds of feet thick in the
Pakokku District, have either never been deposited in the
Magwe District or have been removed by denudation. Thus
a considerable gap in the succession was proved, and the detec-
tion of pre- Irrawaddy movements, as explained above, completed
the chain of evidence. The identification of fossil horizons in
the Pegu Series has since made clear that this unconformability
is often of very great extent, and that great thicknesses of
the upper beds of the Pegu Series have been removed by
denudation in many localities before the deposition of the
basal beds of the Irrawaddy Series.
Another instance of unconformability, for long a matter of
doubt, may be given from Burma, namely, the unconformability
between the basal beds of the Pegu Series and the underlying
Bassein Series, probably of Eocene-Cretaceous age. This line
of discordance had been crossed and recrossed by several
geologists without its being detected, and presumably they
classed the underlying Bassein Series with the petroliferous
Pegu Series above. The first evidence that called attention
to the possibility of there being an unconformity was afforded
by the fact that the great littoral arenaceous group of the Yaw
sandstones, which forms a very strong feature in the foothills
of the Arakan Yomas, is underlaid by softer strata with a
rather different aspect as regards lamination, bedding, jointing
and state of mineralization. Subsequently evidence of move-
ment, folds, and small faults, were noted in the underlying
series and proved not to affect the Yaw sandstones, and finally
the mapping on the six-inch scale of a few square miles,
where excellent sections can be seen, proved that the Yaw
84 OIL-FINDING
sand-stones transgress over hundreds of feet of the Bassein
Series. In many small sections, notwithstanding, the two series
differ so slightly in strike and dip as to appear perfectly
conformable.
In Persia the detection of an unconformability proved of
the greatest importance from the practical point of view of oil-
field development. During the detailed mapping of the oilfield
at Maidan-i-Naphtun the possibility of there being such an
unconformability had been suggested by the discovery of the
detrital limestones and breccias, and the occurrence of great
conglomerates at various horizons in the Tertiary series full
of well-rounded pebbles of limestone. It was known from the
work of previous observers that a great mass of limestone (the
Asmari limestone) lay at a lower horizon, and it had been
suggested that drilling in anticlines of this limestone might be
profitable. When the Asmari limestone was first encountered
by the writer on the north-west pitching end of the Asmari
anticline, evidence of unconformity was at once searched for,
but beyond thin beds of detrital limestone resting here and
there on a somewhat irregular surface of the calcareous rock, the
occurrence of one small patch of breccia, and a slight discord-
ance in dip and strike between overlying beds of gypsum and
the Asmari limestone, no evidence was forthcoming. The
various kinds of limestone preserved in pebbles and fragments
in the conglomerates and breccias near Maidad-i-Naphtun were,
however, matched from the solid outcrop of Asmari.
A few days later, in the next great anticline to the north-
eastward, where the Asmari limestone again appears, demon-
strative evidence at once came to light. A great transgression
of beds high up in the oil-bearing series, chiefly conglomerates
full of limestone pebbles, was observed cutting right across the
anticline of limestone, which in places is entirely removed by
denudation, some 2000 feet of thickness having been denuded.
On the flanks of the limestone outcrop bed after bed of the oil-
bearing series makes its appearance between the calcareous
rock and the great conglomerates which lie across the denuded
anticline.
Subsequent field-work proved that these anticlines were
denuded as they rose under the flexuring movement, and
that the succession may be entirely conformable in the syn-
clines; consequently at the north-west end of Asmari Hill,
GEOLOGICAL STRUCTURE 85
where the fold of lime- stone pitches sharply downwards, no
striking evidence of unconformity could be expected.
The point of importance from the oil-development point
of view is that the oil-bearing strata belong to a different
series, and are of later age than the Asmari limestone, and
to attempt drilling in the latter would be entirely speculative
and unjustifiable. But for the detection of this unconformability
we should be, as far as that region in Persia is concerned, still
in the dark as regards the conditions under which these
Tertiary strata were deposited, and as to what districts are
most favourable for exploitation.
In southern Ohio probable unconformities that do not crop
out at the surface have been detected in strata either horizontal
or very gently dipping, and the transgression is sometimes
exceedingly regular. Mr. Frederick G. Clapp has explained
this matter very clearly in one district, showing that the
Clifton sand (a well-known oil-bearing horizon) increases in
depth eastward at a rate of from 30 to 100 feet per mile more
than is indicated by the datum line given by a characteristic
bed exposed on the surface. In such a case the evidence from
wells becomes more important than a detailed study of the
surface. But such remarkable regularity must be exceedingly
rare, and there is always the possibility that the effect may
be due to lateral variation, the thinning or thickening of the
oilsands and the beds intervening between them and the
surface, rather than to unconformability and overlap. Varia-
tions in thickness as great as this when a series is traced
far in the same direction may be observed in the Sabe and
Yenankyat fields in Burma, where a series of deep valleys
across the strike make it possible to measure the thicknesses
of groups in actual sections. In cases such as that of Southern
Ohio the evidence from wells is essential, and it is a matter
more for the consideration of the oil-engineer than for the
geologist, whose examination of the evidence at the surface
should be complete before wells are drilled in a new
field.
Unconformabilities, the extent and directions of increase
of which have not yet been worked out, are already causing
difficulties to those entrusted with the exploitation of some
areas in Trinidad, and have perhaps done something to con-
firm the popular idea that petroleum is a very capricious
86 OIL-FINDING
mineral, and that, as the driller is fond of reiterating, " the
only way to find oil is to drill a hole for it."
But of all oilfields the successful development of which
depends on a study of unconfor inability and overlap, and the
directions in which erosion of the denuded surfaces beneath
unconformable junctions increase or decrease, the most remark-
able is perhaps the island of Barbados. Here we have evidence
of folding movements of considerable severity acting in different
directions and at different times. There are two great uncon-
formities. The oil-bearing series, much folded and not a little
faulted, is overlaid unconformably by a series of oceanic
deposits, which in their turn have been thrown into flexures,
raised within the zone of denudation, and overlaid unconform-
ably by a thick mass of coral limestone of comparatively
recent date, which rises in terrace after terrace lying hori-
zontally and covers by far the greater part of the island.
The surface of the oil-bearing series is irregular, and there
is an overlap of the upper beds of the Oceanic series over
the lower, while there is another overlap of the coral lime-
stone over the Oceanic series, which has doubtless been
removed by denudation in many places, so that the coral
limestone rests directly on the petroliferous series (the Scot-
land Beds) in several districts. Yet in spite of sharp folding,
faulting and unconformabilities, oil has been produced in small
quantities for several years, and though the work has not been
a great commercial success up to date, there are prospects of
valuable oilfields being proved. Success will depend upon the
working out of the effects of the different movements and con-
sequent unconformabilities, and the determination by such
methods of where the petroliferous strata, whether deeply
buried beneath younger deposits or not, will be found under
conditions most favourable for good productions of oil.
Sufficient has been written to show that unconformabilities
are common phenomena in Tertiary oilfields, and that they
must be studied carefully if the structure of a country is to
be ascertained beyond the possibility of doubt. They are of
great practical importance to any company undertaking
development work.
Thus folds, faults, and unconformabilities must be con-
sidered together and in detail before any connected history
of a country or district can be presented, and it may often be
GEOLOGICAL STRUCTURE 87
necessary to visifc areas far beyond the confines of a district
before some of the problems in structure that it exhibits can
be solved. There must be no such thing as opinion about
geological structure; only the facts will suffice, and the
geologist must make absolutely sure of structure if the drill-
ing programme is to be directed with the least possible
number of failures and the greatest number of successful
results, since in the area selected through knowledge of the
oil-bearing series and its lateral variations it is the geological
structure and nothing else that determines the extent of each
field.
An oilfield with several producing wells, but with no
geological map, may be part of a great potential producing
area, or may be the merest fringe in which oil production
is possible. The area between two producing wells is not
developed or proved, unless the geological structure of the
intervening ground is known, and known to be favourable.
But a very few wells, carefully located, will enable the geologist
to determine within reasonable limits the probable productive
area of a field.
Hence every detail of dip, strike, change in dip or strike,
hade of axes of flexures, and pitch of axial lines must be
noted, and if the area be undulating the height of each locality
where observations have been made about a datum line should
be ascertained. Then and only then can absolute certainty
as to structure be achieved.
CHAPTER VI
INDICATIONS OF PETROLEUM
OUR Manager cables as follows : — " Borehole No. 3 has reached
a depth of 792 feet, and the indications are favourable." To
how many meetings of anxious shareholders have such or
similar comforting words been read, and how often do we see
a message of this nature dealing with a new field under
exploitation quoted in the public press ? And it would be a
very bold and even impudent shareholder who would rise in
his place and ask pointedly : " What are the indications, and
why are they considered favourable ? "
Such queries would no doubt receive answers, but in all
probability they would be vague and carefully guarded
statements, for the Chairman or Managing Director of a
Company may very naturally consider that it is not his duty
to study geological data ; he depends upon the Manager or
Field-Superintendent, who has cabled ; or the log of the well
has been submitted to an expert at home, who has pronounced
the indications "favourable." And the shareholders may go
away satisfied, though it may be that neither Field-Manager
nor expert has any certain knowledge of what would be
" favourable " indications in the locality and at the depth
stated.
This at once raises the question of what are favourable
indications of petroleum, i.e. indications that point to the
probability of good productions being obtained.
The subject naturally divides itself into (1) Surface
Indications, and (2) Indications in a borehole.
(1) Surface Indications. — It is to indications at the surface
that attention has always been attracted. The expert who
visits a new district goes first to the localities where " shows,"
as they are called, are to be seen, and it is largely by the
presence of " shows " in any piece of land that it is judged
88
INDICATIONS OF PETROLEUM 89
by persons without technical knowledge. The field-student
will do well to make himself acquainted as soon as possible
with the nature of the " shows " which he may expect to find
in the country that he is examining. He has, let us say,
made his preliminary traverses, gained some idea of the lateral
variation, and discovered that favourable structures produced
by the earth-movements he has been studying are to be found.
The time now comes for him to study the indications at the
surface as a guide to what thicknesses of strata and what
horizons may be expected to prove petroliferous, and what
variety of oil is present.
Let it be admitted at once that the actual shows of oil
are of great importance, much is to be learnt from them;
but the study of structure must take first place. It is a
surface show that always attracts the lay mind. During the
writer's first examination of an oilfield he inadvertently
grieved an enterprising pioneer who had pointed out a small
seepage with the remark " there is what would make glad the
heart of a Bockefeller," by bluntly answering that he himself
took little interest in such indications as long as the geological
structure was still unsolved. As a matter of fact it is very
frequently where surface shows of oil are seen that drilling
would be entirely unsuccessful, and many of the greatest
oilfields known to-day have not a single surface indication
within their length and breadth.
Surface indications are of various kinds according to the
class of oil, the nature of the strata, and the geological
structure. They comprise : —
(a) Seepages of oil.
(b) Asphalt deposits.
(c) Evolution of gas from gas-pools, mud-volcanoes or
dry ground.
(d) Outcrops of bituminous strata,
and
(e) Veins of manjak or ozokerite.
In addition to these the evolution of hydrogen sulphide
may be in some cases a favourable indication, and crystals of
sulphur in cavities in a rock, or the presence of minute traces
of sulphur in flecks and patches may also be important. Belts
of stunted or sickly vegetation may give a valuable indication
90 OIL-FINDING
where no solid evidence is available. Finally a faint odour
ef petroleum may sometimes be detected where no actual
seepage can be discovered.
(a) Seepages of Oil. — Where an oilrock reaches the surface
there is generally some sign of petroleum. It should be looked
for in low ground, in the beds of streams, or at the foot of
hills, and, if the strata be bent into anticlinal form, at or near
the crest of the anticline. In many cases where the upper
part of an outcrop has lost all signs of petroleum through
weathering, a seepage will be noticed where the outcrop crosses
the valley of some small stream or gully. In such localities
films of oil with a beautiful iridescence may be seen on the
surface of the water. The odour will at once distinguish these
films from decomposing bicarbonate of iron which also gives
an iridescent film (of hydroxide), and which has often been
mistaken for evidence of petroleum. The films in these two
cases, however, are by no means identical, and when seen side
by side could never be mistaken.
If the seepage be more copious, brown or greenish or black
drops of oil may be seen, and these may collect into patches
on the water near their source or in eddies and still pools
down stream. Gas is frequently seen bubbling up through
the water. In some cases actual trickles of oil out of the
rock may be observed. But the greater part of the outcrop
of an oilrock will probably give no indication of being
petroliferous until dug into for a few inches or perhaps
feet.
The cavernous detrital limestones of Maidan-i-Naphtun
exude oil rapidly in the valleys of streams, and where the
water is clear small spherical drops of the oil may be seen
emerging from cavities and rising to the surface. But the
greater part of the outcrop is barren of indications. The
greatest natural show of liquid petroleum which the writer has
seen occurs in this field ; as much as 20 barrels a day of oil
flow to waste in one stream. Three or four brisk seepages
combine to make up this quantity, and from time immemorial
the Shusteris have collected the petroleum and burnt off the
light oils to obtain bitumen.
Another remarkably large seepage occurs in the Trinity
Hills Forest Eeserve in Trinidad. At the time of the author's
visit to this spot a stream some three yards in breadth was
INDICATIONS OF PETROLEUM 91
covered entirely with a dark brown oil with green fluorescence
for a distance of nearly a hundred yards, while gas bubbled up
briskly both through the water and from several places on the
banks. This show is on an outcrop of the Galeota Oil-bearing
Group.
Oils with a paraffin base usually make smaller and less
striking seepages than asphaltic oils, as the results of inspissa-
tion are more readily washed away by rains, and the rock from
which the oil exudes is more easily and quickly robbed of its
petroleum contents under weathering processes. Many of the
outcrop shows in Burma, where the oil is generally light and
full of solid paraffin, consist, even on the outcrops of thick oil-
sands, of very small pools not more than a foot or two in
diameter in the courses of small ravines and stream valleys.
Oil obtained from seepages is always more or less inspis-
sated, and does not give a fair sample of what may be obtained
by drilling, the light fractions having evaporated.
An asphaltic oil can usually be distinguished from a paraffin-
base oil by the manner in which it inspissates ; the former
generally remains liquid or semi-liquid for a longer time but
dries finally to black asphalt ; the latter soon coagulates into
little flakes often of a reddish brown colour, and when present
in quantity and containing much solid paraffin dries into a soft
mass like vaseline, which does not adhere to exterior objects
with the same tenacity exhibited by asphaltic oil, and is con-
sequently more easily washed away.
The most remarkable seepages of oil are those that have
been naturally filtered, and partly or entirely decolorized. In
such cases the petroleum, though it has probably lost its most
volatile constituents by inspissation, has also been deprived of
the bulk of its heavier fractions by filtration. The " white oil "
of Kala Deribid in Persia has already been mentioned ; it is a
limpid mobile liquid that the writer could hardly believe to be
oil till he had dipped his hand in it.
Another interesting example of a filtered oil, this time of
asphaltic base, may be observed exuding from an outcrop of
sandy clays in a small tributary of the Lizard Eiver in the
south-eastern corner of Trinidad. The locality has been called
"Lizard Spring." The oil is dark brown with a green
fluorescence, and it collects on the surface of the water in the
stream bed. When the writer was encamped in the forests
92 OIL-FINDING
near this spot a sample of the oil was skimmed by means of a
leaf from the surface of the water, bottled, and taken into
camp, where it was burnt that night in a small open lamp,
hardly clogging the wick at all. An analysis of the oil
collected in this manner at Lizard Spring was made some years
ago by Professor Carmody, Government Analyst of Trinidad,
who found it distilled like a refined oil, and gave : —
Petroleum spirit ..... 0
Illuminating oil ..... 73
Lubricating oil 25
Residual bitumen 2
100
The specific gravity was '867, and the flash point was above 145
degrees (Abel's test). This oil is sufficiently inspissated to make
it a perfectly safe burning oil, and to contain a fair percentage
of heavy oil and residue. A year or two later a small excava-
tion was made in the outcrop, at the author's suggestion, to
obtain a fresher sample of the oil. Professor Carmody's
analysis showed this second sample to contain : —
Petroleum spirit . . . . .12
Illuminating oil . . . -. . 81*25
Lubricating oil ..... 6
Residual bitumen . . . 0*75
few
The specific gravity was considerably lower. This is obviously
a dangerous oil on account of its percentage of petroleum spirit,
and could not be burnt with safety in a lamp. The two
analyses are interesting as showing the effects of inspissation.
The oils are both well filtered by passage through the
argillaceous strata, and it is hardly necessary to say that were
a borehole drilled at this spot an oil of this class would not be
obtained in any quantity, though heavier unfiltered oils from
the same source would probably be struck.
Evolution of oil is not unfreqently observed in the sea,
where an oil-bearing stratum is exposed beneath the water.
In the Caspian Sea such shows were well known for many years
before any active drilling was undertaken at Baku.
Off the coast of Trinidad there are many places where oil is
occasionally to be seen. Perhaps the best known is just west
INDICATIONS OF PETROLEUM 93
of the famous Pitch Lake, where a brisk evolution of gas with
drops of brown oil may be observed about a quarter of a mile
off shore. The activity of this show varies considerably, but on
a breezy day the locality can usually be detected by the
presence of a patch of smooth water, the film of oil covering it
being sufficient to prevent waves from breaking.
At the mouth of the Vance River, and again at Point
Ligoure, where outcrops of the Rio Blanco Oil-bearing Group
run out to sea, the water is sometimes covered with a film of
oil for a considerable distance. Other submarine shows near
the eastern and south-eastern coasts are sporadic and occasion-
ally of explosive violence ; after an outburst sticky oil and soft
asphalt are washed up on the shore in considerable quantity.
Off the south-western corner of Tobago there is apparently
a submarine outcrop of oilrock, for sticky inspissated petroleum
is washed up on the beach and the coral limestone in great
quantity at some periods of the year.
(b) Asphalt Deposits. — Oils of asphaltic base nearly always
make their presence obvious, when the conditions are favour-
able, by more or less extensive deposits of asphalt along the
outcrops of the petroliferous strata. There is, of course, no
hard-and-fast line between a seepage of crude oil and a deposit
of asphalt; every gradation of sticky and inspissating oil
between the two may be observed on the same outcrop. In
Trinidad, where most, though not all, of the oils are asphaltic,
the phenomena of asphalt deposits can be studied on a remark-
able scale. Foremost of all comes the famous Pitch Lake, the
best known, though not the most extensive, asphalt deposit in
the world. Much has been written about it, and many theories
have jDeen propounded to account for the origin of this lake.
Without going in detail through the theories of various authors
and pointing out where each has advanced the knowledge of
the day, it may be as well to give a brief description of the
field evidence and the last published, and so far accepted,
theory; the author may be pardoned for inserting a lengthy
quotation from his official account from the Council Papers of
Trinidad, Ho. 60 of 1907, more particularly as this account has
been drawn upon extensively by others, and large portions of
it published verbatim and without acknowledgment.
" A brief account of the evidence obtained in the field, and
from other sources, must be given. The Pitch Lake lies upon
94 OIL-FINDING
a well-defined plateau 138 feet above sea level. The area has
recently been affected by gradual upheaval, as proved by raised
beaches in the neighbourhood, and it is probable that the
plateau at no distant date, geologically speaking, stood at or
below sea level, and is in fact a raised beach or coastal beach
itself.
The geological structure is a gentle anticline which runs
roughly east and west, the lake being upon the crest. The
vicinity of the lake is almost entirely covered with surface
deposits concealing the solid evidence. The underlying rocks
are lightly compacted and are often disintegrated to a great
depth, and the surface wash of disintegrated material covers
almost all the ground. The " brown shales " mentioned by
Messrs. Louis and Gordon, though often giving an appearance
of stratification, are not Tertiary sediments, but recent surface
deposits. The brown colour is due to the presence of finely
divided bitumen or asphalt dust.
The La Brea oilsand, a deposit of variable thickness, is the
source of all the pitch. It crops out to westward of the lake in
the coast section, to eastward of the plateau, and also to the
southward near the Vessiny Eiver, and in inliers in hollows.
Its outcrop has been mapped for several miles. This oilrock is
covered by a fine bluish clay, which, when impregnated suffi-
ciently with bituminous material, has occasionally become
ignited and burnt to porcellanite, e.g. south and south-west of
the lake. The clay in its turn is covered by a soft yellow
sand, the disintegrated outcrop of which covers much of the
area north of the lake.
Wherever the capping of clay is thin, or the oilrock is
merely covered by superficial deposit, or is actually exposed,
soft asphalt exudes, forming small cones, examples of which
may be seen beside the road between the Asphalt Company's
works and the lake, and at several places north and west of
the lake.
The oilrock, where it is exposed on the shore west* of the
lake, is a fine dark sand, so full of bitumen that the superficial
layers actually flow slowly, the semi-liquid asphalt as it exudes
carrying the inorganic material of the rock with it. Pieces of
this rock may be twisted off in the fingers and rolled into
pellets. An analysis of a specimen by the Government Analyst
gives the following results : —
INDICATIONS OF PETROLEUM 95
Water, etc., volatile at 100° C.
Bitumen . . ** . , • ..
Non-bituminous organic matter . . .
Ash
Soluble in petroleum ether . * . 8 per cent.
This specimen was taken from a weathered tide-washed
outcrop. The quantity of non-bituminous organic matter is
remarkable, but, as will be seen later, recent work by Mr.
Clifford Eichardson has thrown much light upon this point.
A shallow boring (about 60 feet) was made in the outcrop
of oilrock west of the lake, many years ago. It is situated 200
feet from the sea and yields a small quantity of rather heavy
oil. A sample taken from the surface gave the following results
on analysis by the Government Analyst : —
Specific gravity . . . . . . 0*950
Mineral matter . . . . 0'02 per cent.
On distillation —
Water ./ . . . " . • V 1-2
Petroleum spirit , - . • . . 12*8
Illuminating oil (150° - 300° C.) . 36'0
Lubricating oil (above 300° C.) . . 32'0
Eesidual bitumen . • , . • . • . 12*3
Loss . . . . . ". . 5*7
100-0
In the sea at a distance of about 200 yards west south-west
of the last-mentioned locality, there is an oilspring. A smooth
patch on the water is often conspicuous, and in it drops of
brown oil may be seen floating, while gas bubbles up all round,
and a film of oil sufficient to prevent waves from breaking
sometimes covers the surface for a considerable distance.
In the hollow east of the plateau on which the lake is
situated, the oilrock crops out again, and large flattened cones
of semi-liquid asphalt may be seen with slight evolution of gas.
In these cones or rather pools of soft pitch the material can be
seen exuding, and it is streaky with the quantity of inorganic
matter brought up with the bitumen, indicating that either the
cohesion of the oilrock breaks down when it is exposed, or that
superincumbent material is carried up by the flow of asphalt
and gradually absorbed in it.
96 OIL-FINDING
Borings made by the Asphalt Company in 1893 have
furnished additional evidence of the underlying oilrock. In
the centre of the lake a depth of 135 feet was reached without
touching bottom, but at 1000 feet from the centre on the north
side fine sand was struck at 80 feet, then more asphalt, and
at 90 feet asphaltic sand, i.e. the more or less disintegrated
oilrock. A boring south of the lake also struck a hard
asphaltic sand, obviously the same which crops out to the
east-south-east, the course of which can be traced by lines of
asphalt cones. The oilrock cannot be identified in the coast
section in Guapo Bay, but porcellanite and lignitic shales
covered by sands and sandy clays probably represent it, and
indicate that the oilrock is thinning out and the oil-producing
conditions at this horizon ceasing in this direction.
The next evidence to be considered is the composition of
the lake pitch. This is treated of so fully in Mr. Clifford
Eichardson's book, "The Modem Asphalt Pavement," that a
few brief quotations will suffice. The average composition of
the lake pitch is given as : —
Water and gas . . .29 per cent.
Organic matter, not bitumen . . 7
Mineral matter 25
Bitumen 39
100
The asphalt is an "emulsion" of these constituents. The
inorganic matter consists of fine sand and clay with a small
quantity of iron oxide and soluble salts. Mr. Clifford Eichardson
gives an analysis of the mineral matter as follows :—
Si02 ..... 70-64
A12O3 .... 17-04
Fe203 ; i . . 7-62
CaO . . . ;*** ;... . 0'70
MgO. .« < . . 0-90
Na2O •••. . . . 1-56
K20 . ; . . . 0-35
S03 . * - . . . 0-97
Cl . , ' . . . 0-22
100-0
INDICATIONS OF PETROLEUM 97
This corresponds with the composition of a normal sandstone,
with slight admixture of argillaceous material. The micro -
photograph of the mineral matter which Mr. Clifford. Eichardson
published (" The Modern Asphalt Pavement," p. 34) shows all
the characteristics of the debris from an ordinary fine water-
borne sandstone, the grains not being greatly abraded as in
windblown sands, nor having any of the characteristics of silica
deposited from solution. The finest material is a fairly pure
clay. The percentage of " Organic matter not bitumen "
presents a point of great interest; as recorded above, the
percentage of this in the La Brea oilsand was as much as 29,
while in the Rio Blanco oilsand it was only 0*46, a difference
great enough to enable these different types of oilrock to be
distinguished easily. Kecent work by Mr. Clifford Richardson
upon the absorptive properties of fine clays for bitumen
explains the occurrence of this percentage of hitherto little-
understood constituent in asphalts, oilrocks and manjaks.
In a paper read before the American Society for Testing
Materials, and afterwards published in the "Engineering
Record," he describes experiments made with Trinidad lake-
asphalt and tests of the absorptive and " adsorptive " properties
of various fine clays upon solutions of bitumen. The results
arrived at are briefly that fine clays have the power of
decolorizing bituminous solutions by absorbing or " adsorbing "
a proportion of the bitumen in such a manner that it cannot
again be removed by the action of solvents. Thus the greater
part of the "organic matter not bitumen" can be proved to
be bitumen which cannot be removed in solution. The
presence of water may also have some effect in favouring
this absorption, but the proportion of fine clay present seems
to be the more important factor. Applying these results to
lake-pitch and the oilrock from which it is derived, we have at
once an explanation of the presence of argillaceous material
in the asphalt, and we must increase the percentage of bitumen
in lake-pitch by almost, if not quite, 7 per cent, and the per-
centage in the oilrock probably by a much greater amount.
This makes the breaking down of the cohesion of the oilrock
on exposure much more intelligible.
The lake itself is, by the latest survey made under the
supervision of the Inspector of Mines, 137 acres in extent, the
margins being covered in places by superficial deposits washed
H
98 OIL-FINDING
down from the surrounding ground. In the centre the sur-
face of the asphalt is about six inches higher than near the
sides, and for some distance from the centre there are no water-
channels. Then comes a broad zone characterized by water-
channels dividing the surface into roughly circular areas with
rounded edges. Near the shore the pitch is harder as a rule,
and less cut up by water-channels. Near the centre there
is an area of very soft asphalt, where a little gas issues slowly,
while there are similar but much smaller patches near the
western margin and between it and the centre. The dis-
tribution of these areas of very soft pitch indicates the proximity
to the parent oilrock, whence continuous but minute exudation
of pitch is still taking place. Lest there should be any mis-
understanding upon this point, it must be repeated that Messrs.
Louis and Gordon have proved conclusively that the lake is
exhaustible, and is being depleted at a very rapid rate, but the
presence of the patches of soft asphalt, and the difference in
level between the centre and sides makes it clear that additions
of asphalt, probably amounting to only a few tons in the year,
are still being made, just as the same material is exuding in the
ground to the eastward and south-eastward of the lake.
The gas given off from the lake is chiefly sulphuretted
hydrogen formed by the action of water on sulphur compounds
in the asphalt. It is seen bubbling up in the water-channels.
A small quantity of oil-gas, however, may be detected issuing
from the soft patches/ V •
The " pitch-lands " of La Brea village are undoubtedly, as
pointed out by Messrs. Louis and Gordon, an overflow from the
lake. This overflow has taken and occupied the valley of a
small stream, known as the "pitch-lake ravine," and has in
effect pushed the stream westward, where it now flows at a
higher level than its original course. There is no evidence of
any exudation of asphalt in the village lots, though gas has
been detected issuing from the ground on one or two occasions.
Weathered surface deposits underlie as well as overlie much of
the land-asphalt, proving that the overflow, which has ceased
some years ago, took place under subaerial conditions.
From the evidence detailed in the preceding pages the
origin of the Pitch Lake can be explained as follows : —
In the first stage the La Brea oilsand, covered by its cover-
clay and succeeding sediments, lay below sea-level. Under a
INDICATIONS OF PETROLEUM
99
flexuring movement acting in a north and south direction, the
area was subjected to elevation, a gentle east and west anticline
being gradually formed, and the strata above the oilrock were
raised within the zone of denudation, though probably still
below sea level. Denudation of the crest of the anticline took
place till the reduced thickness of the puddled cover-clay was
not sufficiently tenacious to resist the upward pressure of gas
s.
N.
Sea Level
FIG. 4.— Stage I.
Sea Level
FIG. 5. — Stage II. Submarine mud-volcano.
Sea Level
FIG. 6.— Stage III. Formation of plateau.
Sand,<
Coverclay.
Oil sand s.l
Pitch
FIG. 7.— Stage IV. Present day.
FIGS. 4-7. — Diagrams to illustrate formation of Pitch Lake, Trinidad.
from the oilrock. A mud volcano would be the result, and, as
denudation and elevation both continued, would increase in
size. All this probably took place beneath the water. As the
covering was gradually removed, oil began to exude and to dry
up to a sticky asphalt.
About this time the anticline was probably becoming more
clearly denned, and the site of the pitch-lake began to emerge
from beneath the sea as a hollow in which discharge of gas
ioo OIL-FINDING
and oil was continually taking place, while mingling with
inorganic minerals would be favoured by tides and wave action.
This stage is marked by the formation of the plateau, suggesting
that the" surface remained at or near sea level for a considerable
time.
As the land rose sub-aerial denudation would come into play,
the oilrock itself being exposed over a roughly circular area
defined by the extent of the mud volcano. The anticline being
now well marked, gas and oil would be forced from all sides
towards the crest, where the exposed oilrock would afford relief
of pressure. The bituminous minerals being present in such
quantities in the oilrock as to destroy the cohesion of the
material on exposure, the solid rock would gradually crumble
and flow into the cavity, while lighter oil and gas issuing from
below assisted in the incorporation of the inspissating petroleum
with the detritus of the oilrock and its cover-clay and all other
material washed into the cavity. Thus the basin would be con-
tinually enlarged as fresh strata of oilrock were laid open to
disintegration. Convection currents in the semi-liquid mass
and discharge of gas, while there were still quantities of gas
under pressure at deeper levels, would ensure a thorough
mixing of the different materials into an emulsion. This action
is still going on, but so slowly as to be practically negligible,
while gas and light oils have decreased very greatly in quantity
as the available supply of petroleum became inspissated.
Extrusion of semi-liquid bitumen proceeded to such an
extent that an overflow took place and the valley northward
towards the sea was completely filled with asphalt, which is
still flowing slowly downward, though there has not been any
escape of asphalt from the lake for some years. That this
overflow took place under sub-aerial conditions is proved by the
weathered state of the superficial deposits beneath the land-
pitch, and by the form of the valley floor. This shows also
that the site of the lake had by this time reached a considerable
height above sea level, and sub- aerial denudation must have
meanwhile been affecting the surrounding country, leaving a
remnant of the plateau, but trenching it so deeply on the east-
ward and south-eastward as to expose the oilrock, but not under
such conditions as regards gas-pressure, etc., as to give rise to
mud- volcanoes. The flow and exudation may at one time have
been fairly rapid, but it is now, naturally, very sluggish, owing
INDICATIONS OF PETROLEUM 101
to gradual inspissation. In its latest stage, soil has actually
been washed down from the surrounding country over -the
margins of the asphalt in several places."
The lake is exceeded in size by similar asphalt deposits in
Venezuela, where, however, the bituminous material is purer,
softer, and more difficult to work commercially. The depths of
these Venezuelan lakes have never been ascertained.
Though the evidence of the extrusion of asphaltic petroleum
as afforded by pitch-lakes is very striking on account of the
concentration of the material in one locality, it is no more
significant than the asphaltic deposits that mark the outcrops of
oil-bearing strata in many parts of Trinidad. The deposits are
usually in the form of flattened and rounded cones strung out
in lines along the outcrop, and where no actual exposure of the
strata is seen it is often possible to map the outcrop simply by
these exudations. They vary in size from a diameter of a few
inches up to as much as seven or eight yards, and in height
from an inch up to six or eight feet. Where the exudation is
rapid and copious the cones often coalesce, and an area of an
acre or more may be completely covered with the material.
Similarly a flow of asphalt down a gully may be seen occasion-
ally, though it is seldom that such streams exceed one hundred
yards in length and eight or ten feet in depth.
The consistency of the material also varies from soft sticky
oil to hard compact asphalt that can be broken by the hammer,
the softer varieties being the most recently extruded. The
skeletons and remains of birds and small animals are not
unfrequently found- in su 0'65 . . TO
Organic matter . » . 94'SO . 96'20
Mineral tj . . . 4-55. . 2'80
Percentage of petrolene . 8*80. , 9'6
Specimens from deeper levels in the Vistabella Mine gave
percentages of petrolene up to 15 '2.
In Barbados, where many of the veins do not crop out at
the surface, even higher percentages of petrolene are recorded.
One vein gave 18 per cent, from its columnar selvage and 35
per cent, from the central portion.
The clays surrounding manjak veins are often seen to
contain sticky inspissated oil or liquid asphalt along joint
faces and slip-planes, and nodules of clay-ironstone slightly
more porous than the clay show abundant evidence of impreg-
nation. From the centre of a vein with columnar jointing in
Marbella Mine the writer has seen a semi- solid bitumen slowly
extruding. This material was brittle enough to be broken up
by a sharp tap, but could be bent and twisted without breaking
if pressure was applied slowly. Its percentage of petrolene
was 56 ; it is a later intrusion.
From this evidence it is obvious that the mineral has been
introduced in a liquid or semi- liquid state, and has gradually
dried and hardened in situ. A still more convincing piece of
evidence is the fact that sometimes when a vein is followed to
a considerable depth it is found to end in a sand or sandstone
fully impregnated with sticky oil, "tar-sand" as it is called
in Barbados. This makes the origin of the mineral quite clear,
and its relations to petroleum on a larger scale can usually be
established by field evidence. Thus in the San Fernando Manjak
field an oilsand, with several " shows " of heavy oil on its out-
crop dips steeply beneath the clay beds in which the manjak
is worked (Fig. 8), and presumably underlies these strata
throughout the syncline. The shaded part in the diagrammatic
section shows the zone in which manjak veins have been proved
by mining. It is natural to expect that the crest of an anticline
I
114
OIL-FINDING
would be the most likely place to find veins of manjak, and
small veins have certainly been discovered on or near anticlinal
crests where a considerable thickness of impervious argillaceous
strata lies above the oilrocks, e.g. in the Poole District and near
the "Devil's Woodyard," in Trinidad, but the centre of a
sigmoidal flexure, between syncline and anticline seems to have
WATER LEVEL
FIG. 8. — Diagram illustrating mode of occurrence of Manjak veins in Trinidad
(San Fernando Field). 1. Cretaceous inlier; 2. Oil-bearing sand;
3. Clay; 4. Sandstone; 5. Zone containing Manjak veins.
some special advantages that make it eminently favourable
to the intrusion of these bituminous minerals. Probably the
strains developed during the earth-movement that caused the
flexures have resulted in slip-planes in the argillaceous strata
and so favoured the intrusive action. The pressure of gas
occluded in or associated with the oil was probably the moving
force.
On a minute scale intrusion from the upper surface of an
oil rock may frequently be seen. Where the La Brea Oil-bearing
Group is exposed in coast-section near the Pitch Lake, small
veins of asphalt may be observed extending vertically upwards
from the upper surface of the petroliferous sand, and hand
specimens may even be obtained of such veins not more than
an inch in thickness with portions of the country rock on each
side. This is sufficient to suggest the possibility of similar
intrusions on a much larger scale, given the requisite conditions.
Ozokerite veins occur under much the same conditions, but
INDICATIONS OF PETROLEUM 115
seldom attain to the same size and" thickness that manjak veins
reach.
Paraffin oils being as a rule more mobile and lighter, and
containing less material capable of forming solid residues than
asphaltic oils, are liable to find their way further without
solidifying as they gradually become inspissated, and are not
likely to coagulate in such large masses. Consequently thin
veins and networks of veins, and bands of porous inorganic
material impregnated with the solid paraffin wax are more
frequent than thick well-defined intrusions. The colour of the
ozokerite varies from yellowish white to brown and black, but
the latter colours are by far the most common. The mineral,
being of considerable value, is frequently mined, but the mines
do not often become great commercial successes owing to the
lack of thick and solid veins.
The occurrence of rock-salt or brine-springs is not dealt
with as evidence of petroleum, although the association of brine
or salt with oil is frequent in many parts of the world. In a
former chapter it was shown that this may not be an essential
association, but another indirect effect of the same cause ; con-
sequently though the search for brine has often led to the find-
ing of oil, and its occurrence may often give valuable evidence
to the geologist, it is hardly justifiable to class rock-salt or
brine with surface indications of petroleum.
All the phenomena described above must be noted by the
geologist, and the significance of each learnt, so that he may
be able to ascertain whether or no a, series is or has been
petroliferous; evidence may be very scanty in jungle- covered
ground, and he may have to rely upon very meagre indications,
which might easily be overlooked. It is, therefore, necessary
that every variety of indication should be familiar to him. In
argillaceous strata he must be especially on the alert ; where
exposures are few and dips unreliable, a minute gas-show, or
the discovery of a few fragments of manjak, may be of great
value in assisting him to determine where a test well should be
located.
2. Indications in a Borehole. — Evidence that may be con-
sidered as "favourable," and as pointing to the prospect of
striking oil in a drilled well, may be of almost any nature, and
such evidence can only be interpreted by reference to what is
known of the geological formation or series that is being tested.
n6 OIL-FINDING
During the first tests of a new presumed oilfield, where perhaps
little or nothing is known of the geology of the district, a state
of things which even nowadays may be met with only too
often, " shows " of gas and oil are really the only favourable
indications that can be recorded. And even these may be
entirely deceptive, for it may be that such shows are derived
from horizons which in other districts are represented by thick
and prolific oilrocks, but which have thinned out to insignifi-
cant streaks in the area being tested. And the driller may be
tempted to drill deeper and deeper into strata that are not and
never have been petroliferous. The well may even pass through
an unconformability into some lower series, which, if exposed
at the surface, would never be tested for petroleum even by
that most hopeful of optimists, the driller of wild-cat wells.
Yet, because light shows of oil and gas were encountered at
some stage or stages, the well may be continued for months at
ruinous expense.
On the other hand, when the geology of a district has been
carefully worked out, when the strata to be drilled through
are known, and the depth to be drilled estimated approxi-
mately, a " favourable indication " consists of any evidence that
shows that the strata to be tested are being approached, and
the fact that no shows of oil or gas are encountered may be a
favourable indication, proving that the petroliferous contents of
the strata beneath are securely sealed beneath an impervious
cap and that migration upwards has been prevented.
The recognition of any known baud of rock in the log of
the well, or by fragments from the bailer, even if it be a prolific
water-sand, which will enable the depth to the oil-bearing
horizon to be re-estimated, is often of great importance, as
where lateral variation in rock groups is the rule estimates of
thickness made from some section at a distance can never be
very accurate.
When one or two wells have been drilled, information from
the boring journals should be sufficient to enable the geologist
to judge whether the prospects of a third well are promising or
not, and the depth can be calculated with a fair degree of
accuracy if the area has been geologically mapped. But with-
out a large scale geological map the boring journals are of very
little use unless the wells are close together. In the author's
experience estimates of the depths to be drilled have come
INDICATIONS OF PETROLEUM 117
within 10 feet of the actual depth in the case of new wells two
miles distant from any previous well, and for depths of nearly
2000 feet, in an area of great and sharp flexures. Such
estimates were arrived at by careful six-inch mapping and
making allowance for the thinning of rock groups owing to an
ascertained lateral variation.
Oil is seldom struck without any warning ; light gas
" shows " or light shows of filtered oil and gas often occur at
some distance above the actual oilrock. These are due to a
gradual migration from below. Gas is not necessarily a hopeful
indication, but „ when gas-pressure increases steadily as the
drill penetrates deeper and deeper into a fairly impervious
group of strata, it may be taken as a very favourable sign ; the
first porous band of any thickness met with will probably be
oil-bearing. Even in such a case, however, the oilpool may be
missed and the oilsand found to be full of water. An example
of this occurred in Trinidad. A light show of oil was struck at
shallow depth and cased off; the well was continued and struck
strong gas in a sandy shale. The gas-pressure continued to
increase as the boring proceeded, and caused much difficulty in
the drilling. At greater depths, however, the gas-pressure
began to decrease, and when the well reached a thick sand-bed \
it was found to contain salt water. The gas had reached the ]
locality by lateral migration.
It is, of course, when the first tests of a new field are being
drilled that indications become most important, and especially
when unknown strata are being penetrated. Though a district
may be mapped geologically with great care, and the series
proved to be petroliferous, the first well may be drilled into
strata that are not exposed for a distance of many miles from
the locality. A study of the lateral variation may have made
it appear highly probable that oil-bearing strata are beneath
the surface, the geological structure may be eminently favour-
able, and the well carefully located, but, as the depth to be
drilled is unknown or only roughly estimated, there is neces-
sarily some uncertainty. It is in such circumstances that the
evidence from the log must be most carefully studied. Any
light show of gas or oil, if in thin beds, will be a favourable
sign. But if thick porous beds are pierced with light shows of
gas or oil accompanied by water, the indication is most un-
favourable. If the drill has passed through a great thickness
u8 OIL-FINDING
of stiff argillaceous strata, when it first reaches a porous bed
important evidence will be forthcoming ; if oil appears in the
bed the indication is most hopeful, but if water, the prospects
of the well are gloomy. The nature of the argillaceous strata
have also to be considered ; if they are typically marine through-
out, the prospects will not be quite so good as if estuarine con-
ditions are indicated by the presence of gypsum or selenite at
some horizons, and especially towards the base of the argil-
laceous group.
Alternating bands of clays and sandstones may be regarded
as moderately favourable, even if the sands contain water.
Nodules of clay-ironstone,calcareous concretions in sandstones,
glauconitic sands, and all the characteristics of estuarine and
deltaic beds may be regarded as favourable.
Beds of coal or lignite, if pierced at comparatively shallow
depths where comparatively thick clays underlie them, are
hopeful indications if the geological structure be good; if
struck at great depths, the field will probably have to be
abandoned.
Beds of gypsum or rock-salt are indifferent evidence; oil-
bearing strata are not infrequently found below them, but just
as frequently above them, while in many cases they are not
associated in the same series with petroleum.
The occurrence of marine limestone is, generally speaking, a
bad sign, though many prolific fields have a limestone as their
reservoir rock. An entirely marine series, without intercala-
tions with littoral or estuarine beds, is to be avoided.
Fresh arkoses or grits containing fresh felspars, micas or
volcanic material, are usually unfavourable as indicating the
proximity of crystalline rocks or volcanic strata which were
being denuded while the series was being deposited. The
approach to an unconformability, however, which may be
indicated by the presence of conglomerates formed of pebbles
derived from an older series is often worth noting, as the basal
arenaceous groups of a series are frequently oil-bearing under
favourable conditions. The reason of this is obvious when we
consider the landward margins of a delta, and the probability
of the formation of swamps between the main mouths of a
river and the higher ground that may bound the delta on one
or both sides. Pebbles or fragments of pebbles may frequently
be brought up in the bailer, and a bed consisting chiefly of
INDICATIONS OF PETROLEUM
119
pebbles can be recognized by any competent driller, so there
should be no difficulty in ascertaining the presence of con-
glomerates.
If a thick arenaceous series, whether conglomeratic or not,
is being drilled, and salt water is found in it, there is little
hope of an oilwell till some underlying impervious rock group
is reached and drilled through.
FAVOURABLE.
UNFAVOURABLE.
Always.
Usually.
Sometimes.
Usually.
Always.
Shows of oil
Shows of fil-
Shows of oil
Light shows of
with strong
tered oil with
with very
oil in thick
gas in thin
'gas.
little gas.
porous beds
porous beds
with water or
among imper-
brine.
vious strata.
Evidence of
Evidence of en-
estuarine or
tirely marine
deltaic con-
conditions.
ditions.
Beds of gyp-
sum or rock-
salt.
Brine.
Shows of gas
<
below or in a
thick argilla-
ceous series.
Shows of par-
Shows of par-
tially inspis-
sated oil near
- -
tially inspis-
sated oil deep
the surface.
down.
Water - sands
below a thick
argillaceous
series.
Lignites or
coals, fossil
Sulphuretted
hydrogen ac-
Hot water with
neither oil nor
resin, sul-
companied by
gas.
phur or sul-
hot water.
phure tted
,
hydrogen.
Gas in slightly
Gas-shows ac-
porous strata,
companied
with pressure
by water in
increasing
porous beds
downwards.
among im-
pervious beds.
Ozokerite or
manjak veins.
120 OIL FINDING
But when all is said and done, every case must be con-
sidered on its merits by reference to what is known of (1) the
geology of the district or country ; (2) the stratigraphy of the
series that is being tested, and (3) the geological structure in
the particular locality. An indication may be exceedingly
favourable where the structure is not very attractive, while in
a field with ideal geological structure it might give by no
means a hopeful prediction as to the results likely to be
obtained.
It is therefore almost impossible to tabulate what are, or
are not, hopeful indications, and the table on p. 119 must be
regarded only as a rough guide to the geologist who has to
study well records in a new field. It is presumed that the well
has been located where the geological structure is favourable.
CHAPTER VII
STRATIGRAPHY
IN the foregoing chapters an account has been given of the
principal subjects which the prospecting geologist must study
in the field before he will be thoroughly competent to advise
a company in the exploitation of petroleum. The necessity of
elucidating lateral variation has been dealt with, the working
out of geological structure has been treated at some length, and
the various kinds of evidence upon which a series can be
determined to be petroliferous have been described. But this
is not sufficient ; the geologist must leave little or nothing to
chance or guess work. It remains to correlate the facts that
have been collected and to get at least a general grasp of the
stratigraphy of the country or area examined.
This is not a matter of merely academic interest, but is of
very practical utility, for though the directions of variation
may be known, though the petroliferous nature of the series be
assured, and though an exceedingly favourable geological
structure be discovered, there may not be any oil-bearing rock
of importance beneath the surface within reach of the drill.
Thus, in Lower Burma a well might be located on a good anti-
clinal or dome structure, among what used to be known as the
"Pfome Series," which is undoubtedly petroliferous, and on
drilling being commenced the well might very shortly penetrate
into the Sitshayan shales, a marine group of great thickness
which has never yielded petroleum. Or again, in either Lower
or Upper Burma an area of excellent structure high up in the
Pegu Series might be tested where the depth to the nearest
petroliferous bands might be so great as to make it impossible
to reach them, or if possible, at an expenditure of time and
money that would effectually prevent the field from being
remunerative. Instances of failure under such conditions are
only too frequent, and similar cases can be mentioned from
121
122 OIL-FINDING
Trinidad, Persia, and Baluchistan. In fact the writer, even
with his limited experience of oilfield exploitation, has come
across cases of failure owing to neglect of stratigraphical study
in every field with which he is intimately acquainted.
It is essential, therefore, that the main stratigraphical
groups of a series should be determined, and the geologist must
be able to recognize within reasonable limits the position in the
series of any horizon that he has to study.
In the course of field-work the geologist will necessarily
gather a great number of facts of stratigraphical importance,
especially during his study of the lateral variations, for it is
those variations which complicate the issue, and make the
establishment of a stratigraphical sequence a matter of no small
difficulty. A correlation or tabulation of the facts is necessary,
and as each new area or district is examined something will be
found to add to or modify the correlation previously attempted.
Finality, if the area be large, is almost impossible to attain, but
the broad lines may be laid down to be improved, modified, or
confirmed by future observers.
Where sections through the entire series in which oil occurs
are to be observed, the geologist will do well to examine them
as soon as possible ; he then starts with a sound basis for
generalizations. Measurements of the thicknesses of groups of
different types of sediment should be made wherever possible
and noted for each particular district. Such measurements
need not be made on the ground if evidence be abundant and
the area be mapped carefully on a large scale ; sufficient
accuracy will be assured by measurements on the map.
Vertical sections of the strata observed should then be con-
structed for each district or locality.
Lithological characters must be studied closely, but too
much reliance must not be placed on them for purposes of
correlation ; for if variation be rapid, precisely similar conditions
of deposition will be found to have occurred at different epochs
in different areas, and may occur again and again in the same
area. Thus almost any particular variety of strata may occur
at almost any horizon in a thick series, and to found any
generalization upon resemblance in lithological characters,
unless the rocks can be actually traced along the strike from
one area to another, may lead to fatal mistakes.
State of Mineralization. — In a thick series, however, there
STRATIGRAPHY 123
are some points that may be noted with great advantage, and of
these first of all comes what may be generally expressed as the
" state of mineralization." When one is dealing with a series
of from 5000 to 10,000 feet of strata— and the prospecting
geologist will probably have to study a mass of sediment
somewhere between these limits— it is only natural to expect
that the older deposits have been more greatly affected than the
younger by the conditions of temperature, pressure, and circula-
tion of underground waters to which they have been subjected,
and the longer period during which these conditions obtained.
Thus harder and more compact strata will be observed among
the lower horizons than among the upper, even when the
sediments are of practically the same composition. Jointing,
again, will be more perfectly developed in the older strata,
especially in the argillaceous rocks, which are more susceptible
to pressure than arenaceous rocks. Thus a concentric weather-
ing and exfoliation may be prevalent among clay groups in the
lower part of a series, and altogether absent from similar clays
among the higher horizons. The formation of veins, whether of
selenite or calcite in clays, and the slickensiding of these veins
owing to minute movements, is another point to be noted.
Ceteris paribus, these are always more conspicuous among the
older horizons. If the series contain lignites or coals, they and
their underclays usually furnish easily recognizable evidence,
the tendency being for the older carbonaceous deposits to have
become harder, blacker, better jointed, and, as proved by
analysis, to have lost water to a greater extent than the younger,
while the underclays develop at least the rudiments of
stratification, which they may not exhibit till subjected to con-
siderable pressures.
Among arenaceous strata the solution of iron compounds or
calcium carbonate, and their redeposition in cementing laminae,
or concentration into concretions, are effects which have required
time as well as the necessary conditions as regards pressure,
temperature, and presence of carbonated water; so younger
strata may give very little evidence of such action, the effects
of which are common enough in strata of greater age.
All these minor points, none of which is of great importance
in itself, may by their cumulative evidence enable the field-
student to detect the difference between a lower or middle
horizon and an upper horizon in the series, or between upper or
124 OIL-FINDING
middle and lower horizons, so that in dealing with a consider-
able thickness of strata, exposed in an inlier and perhaps un-
conformably overlaid, some idea may be at once suggested as to
the position in the series of the horizons exposed. It must be
remembered that these points of enquiry are of chief value in
Tertiary strata, where the youngest rocks are very little altered
since their deposition. Where folding has been intense, and on
a large scale, the mineralization of strata has naturally pro-
ceeded further than in undisturbed regions, and this must be
taken account of when comparing strata from different areas.
Alteration in Character of Sediment. — Frequently, also,
it may be found that the detritus from which sediments are
formed has altered in character as the series is ascended ;
pebbles of some particular rock may be found in the upper or
lower beds alone ; if this is found to hold good over a wide area
it becomes of great importance as proving that different strata
were being denuded at different times. Thus in the Yaw sand-
stones at the base of the Pegu Series, pebbles of agate are
frequent in some parts of Upper Burma, but throughout the
rest of the series they are absent, and it is not till the post-
volcanic stages of the succeeding and unconformable Irrawaddy
Series that agate pebbles are again observed in the Tertiary
sediments. Similar instances could be given without number,
but this will be sufficient to illustrate the point that the con-
stituents of an arenaceous group may on occasion furnish a
clue to its age.
Details of this kind may be noted on the vertical sections
made for different districts, and may be of great help in
establishing the stratigraphical relations of different groups.
Fossil Evidence. — Of all aids to correlation and the working
out of a stratigraphical sequence that will hold good over large
areas, there is nothing more valuable than fossil evidence, pro-
vided that it is abundant, and that it is made use of in a
practical way, as the handmaid rather than the mistress of
stratigraphy. Let no practical geologist take upon himself to
despise the evidence that he may glean from fossil fauna. Their
collection and study may entail a great deal of extra trouble,
and many a weary day spent indoors, but any definite results
obtained are certain, and may enable correlations to be made
that cannot be accomplished by any other means.
STRATIGRAPHY 125
The writer confesses to have little patience with the
zoological side of palaeontology, and even to be indifferent as
to the name that may be given to the fossil part of any particular
organism, but he has had experience of what can be done in
the way of correlating isolated and far-separated areas by the
careful mapping of fossiliferous horizons and the collection of
their fossil organisms, even where faunal change in time is slow
and the species many and often ill-preserved.
Even if the field-student be not interested in palseontological
work, even if he be ignorant of the generic names of the
organisms, he will do well to collect and label them carefully,
and note on his vertical sections the horizons from which they
were obtained. Let him call them " Tom, Dick, and Harry "
if he will, so long as he can recognize them again and can
point to the horizons from which they were collected.
It may be of interest, and of use also, to the petroleum
geologist if a brief description is given here of the methods of
handling palseontological evidence, originated and put in practice
by the Burmah Oil Company's Geological Staff.
In Burma one of the chief difficulties is in the correlation
of different fields, as the petroliferous Pegu Series appears
frequently in widely separated inliers. These inliers are
overlaid unconformably by the fluviatile Irrawaddy Series,
during or previous to the deposition of which there was
extensive and sometimes very great denudation of the under-
lying strata. Thus the local base of the Irrawaddy Series may
be found resting upon almost any horizon in the Pegu Series,
and measurements downwards from the base of the upper series
are useless as an aid to the determination of horizons in the
Pegu strata as a whole. The amount of pre-Irrawaddy
denudation varies greatly within short distances. Added to
this there is a lateral variation in the Pegu Series so great
that correlation of areas by a study of lithological characters
is practically impossible, unless the areas are close together,
while the main groups of strata in any field thicken and thin
out with bewildering rapidity, so that the whole series, even
where the upper part is not removed by denudation, varies
greatly in thickness in different districts.
The area examined by the Geological Staff* of the Burmah
Oil Company up to date is approximately 20,000 square miles,
most of it, however, covered by younger deposits than the
126 OIL-FINDING
petroliferous series. The Pegu Series at its greatest development
reaches a thickness of at least 10,000 feet. It will be readily
understood that the problem of working out the stratigraphy, so
that the thickness of strata containing petroliferous beds should
be known in each district, and the horizons exposed in each
inlier identified, presented many difficulties.
Luckily there was a considerable mass of evidence from boring
journals available, but it would have been of little value without
palseontological evidence, which is abundant, almost every inlier
containing at least one and generally two or three rich faunas.
Some means had to be devised to enable correlations of
isolated areas to be made, and palseontological evidence, if
available in sufficient quantity, was obviously suggested. It was
ten days of continuous rain when encamped in a very fossili-
ferous district that first turned the writer's thoughts towards an
enquiry as to whether there was sufficient f aunal change through
the Pegu Series to allow of its being subdivided into zones.
Dr. Noetling of the Indian Geological Survey had published
seven years previously a memoir (" Palaeontographica Indica,"
A7ol. I.) on the fauna of the Burmese Tertiary rocks, describing
and figuring two hundred and eight species, and attempting a
stratigraphical arrangement of the sections examined up to
that time. Comparatively little of Burma had been gone
over by then, and many of the fossil collections were made by
previous observers, the localities from which some of the
faunas were obtained were but vaguely known, and the relative
positions of different beds in the series were uncertain. Thus
in spite of the ability with which Dr. Noetling marshalled
the evidence, he was handicapped at the start by mistakes in
stratigraphy inevitable when no large-scale mapping is done.
The mistakes were also made of referring every section examined
in Burma to a type-section in the Prome District, of arbitrarily
subdividing the series into a supposed fossiliferous and non-
petroliferous upper division and a petroliferous and non-fos-
siliferous lower division, and of treating the local faunas as
" zones." Every geological observer who has done work in Burma
since the publication of Dr. Noetling's Memoir, and has started
with it as a basis, has helped to bring about almost inextricable
confusion, from the effects of which the official work of the Indian
Geological Survey in the Burmese Tertiaries is only beginning
to emerge.
STRATIGRAPHY
127
The Geological Staff of the Burmah Oil Company when
driven perforce to study fossil evidence in the attempt to
correlate different areas, began by making vertical sections of
each field surveyed, marking the horizons of each fossiliferous
bed and each oil-bearing band. Mapping was usually done on
the six-inch, but occasionally on the eight-inch scale, so it
was possible to make fairly accurate estimates of the thicknesses
of strata exposed. The faunas collected were then compared with
Noetling's faunas treated as if they were " zones," but arranged
in a somewhat different order from that published in the
" Palseontographica Indica," as it very soon became apparent
that some modification in his stratigraphical arrangement would
have to be made.
Areas where many fossiliferous beds at different horizons
were found soon demonstrated that there is considerable faunal
change in time relation in the Pegu Series, and a number of
rough graphs or diagrams were made use of to illustrate this,
still using Dr. Noetling's " zones " as a basis. The " zones "
FIG. 9.
FIG. 10.
A, B, C, etc. were arranged vertically, and distances Aa, B6,
etc. were measured off horizontally in proportion to the number
of species common to the zone in each bed (Fig. 9).
Joining the ends of these horizontal lines we get a figure
dbcdef, which in the case illustrated indicates that the fauna of
the bed under examination resembles the fauna of zone C most
closely, but is probably somewhat higher in the series. The
128 OIL-FINDING
method is rough and open to many objections, but if the zones
contain a sufficient number of species, and the fauna to be
examined is a rich one, an ambiguous diagram such as Fig. 10,
which leaves it doubtful whether the bed should be placed near
the top or the base of the series, is hardly possible. Another
advantage is that the breadth of the diagram shows at once if
the fauna be a rich one or not; the greater the number of
species in a bed, the more weight is naturally given to the
evidence obtained from it. .
Many difficulties had to be encountered. For instance, the
discovery of species not described by Noetling tended from the
first to complicate matters. This difficulty was partially got
over by procuring such books of reference as were available :
Dr. Martin's beautiful figures and descriptions in " Die Fossilen
von Java " proved of great assistance.
Then it was found that Dr. Noetling's zones did not make a
satisfactory basis, some of his faunas are littoral, some laminarian
or pelagic, and some indicate brackish water conditions ; some
are very rich in gasteropods with few lamellibranchs, and
others contain numbers of lamellibranchs while gasteropods are
poorly represented. It became evident that true zones were
required, not faunas from single beds.
Several sections were mapped and measured from the base
to the top of the Pegu Series, and all fossiliferous beds in them
carefully collected. It was possible then, by a comparison of
the vertical sections, to combine them and place many of the
faunas in their true relative positions. The measurements of
the thicknesses of groups were of great use in this, though
owing to the thinning and thickening of different parts of the
series no fossil bed could be placed in the series entirely on
such evidence, unless it was confirmed by palseontological data.
A range- table was tentatively constructed ; it contained
between 280 and 300 species arranged horizontally, while the
horizons were arranged vertically. A small diagonal cross
marked each occurrence of a species, and every occurrence of
the same species lay upon a vertical line indicating the range
of the form so far as it has been ascertained. The majority of
the forms dealt with were gasteropods, which were not only
more easily identified than the lamellibranchs, but which
seemed to show more marked changes in time relation. The
lamellibranchs, however, proved of great value, though the
STRATIGRAPHY 129
ranges of some forms are apparently very long. Echinoderms,
Crustacea, and corals proved very useful, but they do not
always occur in sufficient numbers in the littoral beds which are
usually the most fossiliferous members of the Pegu Series.
Fish, mammals, foraminifera, and one solitary brachiopod have
been made use of ; in fact, the occurrence of every organism
found was recorded on the range-table.
Dr. Noetling's faunas were made use of after being placed
among the faunas as nearly as could be ascertained in their true
stratigraphical positions.
After the range-table was constructed, it was divided
horizontally into seven zones by means of horizontal lines at
convenient intervals.
When any new fauna was discovered, and the species
identified, it was compared with the faunas of the several zones
by means of the graphs or diagrams mentioned above. There
was very seldom any doubt as to the zone to which a fauna natur-
ally belonged. Having ascertained the zone, the fauna was
then compared with the chief faunas in the zone and its relative
position towards them ascertained, and if there was no doubt as
to its position the new fauna was at once put upon the range-
table. Comparison of vertical sections was of great assistance
in the placing of a new fauna.
The first "range-table served very well, but it soon became
out of date. Faunas from all parts of Burma were constantly
being brought in, and great numbers of entirely new species
came to light. In fact, evidence accumulated so rapidly that
additions and modifications were constantly being made. The
attempt to reconcile carefully-measured sections with Dr.
Noetling's stratigraphical arrangements proved a matter of
difficulty, and after the same sections that he describes had
been carefully mapped, new vertical sections were substituted
for his, and it was decided that when a new range-table had
to be constructed Dr. Noetling's faunas should be omitted,
and the stratigraphical arrangement of fossils based upon the
four or five complete sections from base to the top of the series
which were available.
The new range-table deals with approximately one hundred
rich faunas and numberless beds containing only a few species.
The number of species and distinct sub-species or variations is
between 450 and 500. The work, the magnitude of which only
K
130 OIL-FINDING
gradually became apparent, can perhaps never be complete ; the
collection in the Company's geological office numbers many
thousands of specimens, nearly all of which are in a good state
of preservation. All doubtful identifications have been rejected
and are not entered on the range- table.
The length of range of many of the forms, and these often
among the commonest, has proved disappointing, but in this
the accumulation of material has been of benefit, as with more
and better specimens it is often possible to detect variations
and to split a species into two sub-species, one being character-
istic of the lower part of the series and one of the upper.
The occurrence of certain types in certain provinces and
apparently not in others was one of the initial difficulties, but
this has gradually yielded to the effect of more and more
evidence being brought forward : the fauna of the Pegu Sea at
any epoch seems to have been fairly constant over the area in
which it has been studied.
The series is now divided into five main zones, and further
subdivision is possible. Though seldom more than sixteen or
twenty species are found only in one zone, the ranges of many
of the forms are sufficiently short to be of great stratigraphical
value. Any mixed fauna of twenty to thirty species can usually
be placed without any difficulty in its true stratigraphical
position, and a difference of 200 feet in horizon between two
rich faunas can be shown by diagrams. The collection of fifty
species from one bed is by no means a rare occurrence in
Burma.
The methods employed in dealing with such a mass of
palaeontological evidence are at the best rough and ready, and
may not commend themselves to palaeontologists generally, but
the practical use of fossil evidence for practical purposes con-
nected with oil development has been the point always aimed
at. Many species have no doubt been incorrectly named, many
even of the genera may not have been determined beyond the
possibility of doubt, but the ten thousand feet of the Pegu Series
have been divided into zones which have held good up to the
present, the effects of lateral variation have been abundantly
proved, the advance of the delta has been determined beyond
the possibility of doubt ; it is possible to state with a fair degree
of accuracy what zones in each district will be petroliferous and
and what zones barren, and every bed with a rich fauna can be
STRATIGRAPHY 131
placed in the series within one or two hundred feet of its true
position, and datum lines for the correlation of any new field
are furnished. Many details also of the geology of Burma,
details of which it would not be in the interests of the Burmah
Oil Company to permit the publication at present, have been
brought to light.
It is not intended that this palaeontological work of the
Burmah Oil Company's Geological Staff should be taken as an
object lesson by the field-student ; this brief account of it has
been given merely to show what practical value can be derived
from the study of fossils by a staff, none of whom would claim
to be specially qualified as a palaeontologist. It is urged upon
the geologist who is engaged in oilfield work to collect such
fossils as he may find, and to label them carefully for future
study. They may prove of great assistance at some future
day, although apparently of little interest or importance at the
time that they were collected. So long as palaeontology is kept
in its proper relation to field-mapping, so long as generalizations
are not founded upon the sporadic occurrence of a few species*
but on evidence from a large thickness of strata and a wide
range of organisms, every fact that can be brought to light and
tabulated will be of service.
The idea of a range -table is to safe-guard against sources
of error, for a few wrong identifications from a bed containing
many species may not affect the final result appreciably. The
more species collected and identified from a bed, the more
certainty will be attained in assigning it to its stratigraphical
horizon. It is a very old proposition, but never more aptly
illustrated than in palaeontological work, that a correct con-
clusion is more easily reached by considering a great number
of minor points, none of which may be in itself of supreme
importance, but the cumulative effect of which is great, than
by seizing upon three or four salient facts and founding a
generalization upon them.
CHAPTER VIII
LOCATION OF WELLS
ANY practical operator, manager, field-superintendent or driller,
who has been good-natured enough to read so far in this little
book may well exclaim " Now at last we come to practical
politics : what can this geologist tell us about the locating of
wells ? " With the spirit of such a reader the writer heartily
agrees. All that has gone before is leading up to this one
important matter, the choosing of the sites for oil wells, so that
the oil-bearing strata may be struck with a minimum of trouble
and expense, and under conditions that should yield a maximum
production.
It is, of course, in the case of the first test-well of a new
field, or presumed field, that the importance of carefully select-
ing a site is most forcibly brought home to us, and it is this
aspect also which appeals most to the general public. The
geologist who undertakes oilfield work will soon weary of the
oft reiterated question, " How do you know where to put a
well ? "
There are many methods of actually making a first selection.
It is told of one well-known and very successful exploiter and
driller in the United States that he frankly stated that his
method was to put on an old and cherished hat, and to gallop
a rough horse about the country side or farm till the hat
dropped off. On the spot where it fell he drilled the well.
The story is at least ben trovato, and it is possibly quite
true.
The writer knows one highly productive and very valuable
field, miles from the nearest surface indication, where the first
test-well site was selected in almost as haphazard a fashion.
Drillers and field- superintendents had met to make the location,
and the area in which a spot was to be selected was generally
determined, but with characteristic caution none would venture
132
LOCATION OF WELLS 133
an opinion before the others as to what exact spot should be
fixed upon. At last, one bolder spirit than the others, spoke
up and said, " Well, boys, if it's all the same to you, let's put
the well where that crow sits down," pointing at the same
time to a crow which was flying about them. The crow
alighted, the spot was marked, and the well drilled with
remarkably successful results ; it is still producing after
eleven years. A flight of a hundred yards or so further to
the eastward would have put the well beyond any hope of
striking oil.
Well-sites, in fact, have been selected for very many
reasons ; the colour of the soil, or the proximity to an oilshow
have frequently been responsible for the erection of a derrick
at a particular spot. The divining rod has been utilized
occasionally, and sometimes with successful results, while
complicated instruments have been invented and put on the
market to enable any one to detect the presence of oil beneath
the surface, but as to whether or no there is any scientific
basis for the working of such instruments the author must
plead ignorance.
But the ordinary workaday geologist must not depend on
quasi- supernatural aids nor little understood inherited instincts.
By his geological map he must stand or fall, for he will soon
appreciate the fact that, however good and careful his work
may be, it is upon the wells that he locates, especially in new
fields, and upon the results obtained from them, that he will
be judged. An error of judgment made, a fact lost sight of,
a calculation not checked and rechecked, an allowance not
made for some condition that may be inferred but cannot be
observed, and the well may prove a failure, with the effect that
his reputation as a practical man may suffer undeservedly.
The popular idea that petroleum is a very capricious and
uncertain mineral, and that the only way to be sure of finding
it is to drill a borehole, is rapidly dying out, but still it is not
possible to drill for oil with the same confidence with which
one can drill for water. It is often impossible to be sure
whether there is petroleum beneath the surface or not, but
fortunately it often is possible to be quite certain that oil will
not be obtained by drilling.
When the series has been proved to be petroliferous in the
particular district, when the stratigraphy has been worked out,
134 OIL-FINDING
and it is known that oil-bearing horizons are within reach of
the drill, and when the geological structure has been proved to
be favourable, the striking of oil becomes almost a matter of
certainty. In such a case, when the geological map on a large
scale has been completed, the locality for the first test-well is
indicated beyond a doubt, and it only remains to select the
most convenient spot for access, transport, water supply, etc.
within that locality.
But though the map shows the exact spot that should give
the most favourable results, it will be the task of the geologist
to find that spot on tJie ground, and to see that no mistake is
made in marking it. This is the more important as the
first geological survey, though entirely correct as regards
structure, may not be accurate as regards topography ; the well
must therefore be located according to the geological structure
rather than according to the topography.
In symmetrical domes and anticlines a position upon the
highest point of the crest is indicated, that is to say, where the
crest reaches a maximum height geologically speaking, and
probably quite independent of the surface contour of the ground.
Towards such a locality oil and gas will naturally tend to
migrate, gas pressure should be highest, and production greatest.
But even in this case it may be -advisable to select a site
slightly removed from that theoretically indicated, for purely
practical reasons. Where gas-pressure is likely to be very
great and the structure is favourable to a great concentration
towards a crestal point, a well might encounter great difficulty
by striking a violent discharge of gas before any oil is reached,
and it might be necessary to allow the gas to blow off for
months before drilling could be continued into the oilrocks,
and oil produced in any quantity. If the gas could be con-
trolled and utilized at once, there could be little objection to
the drilling of such a well, but this might not be possible in
the circumstances, and much valuable gas-pressure might be
dissipated before the well could be brought in. In such cases
the well often drills itself in, but this is seldom a satisfactory
result, as the handling and anchoring of casing may be pre-
vented by the rush of gas and oil. A well, however, located
slightly down the pitch or the flank of the flexure is not so
likely to meet with the same difficulty, but will probably be a
producing ^oilwell ^as soon as the oilrock is reached, so that
LOCATION OF WELLS 135
a much better idea of the capabilities of the field will be
obtained without delay.
It is possible that in very few cases is gas stored at the
crest of an anticline to the exclusion of oil, but it is quite
probable that gas may be struck before the oilrock is reached,
and the pressure may be great enough to cause damage to
plant, if not even loss of life, when a well suddenly taps such
an accumulation of gas under very high pressure.
In those cases where large mud-volcanoes occur on the
crests of anticlines, similar precautions must be taken in locating
the first well. High gas-pressure and possibly flows of mud
may make the drilling very difficult, if not impossible, on the
crest of the flexure, while a well slightly down the flank may
not suffer from the same disadvantage. The geologist must
judge from the results of wells drilled under similar conditions
in the same country, or from his experience in other countries,
whether there be any danger of a well proving troublesome in
this manner.
When the dome or anticline is asymmetrical the well must
be placed not on the crest but on the flank on which the gentler
clips occur. This is owing to the hade of axial plane of the
flexure, and the reason is obvious when a horizontal section
through the fold is drawn to scale. Mr. E. H. Pascoe has
explained this point very clearly in the " Records of the Indian
Geological Survey," Vol. XXXIV., Part iv., 1906, where he
gives a formula by which the distance from the crest at which
a well should be located on an asymmetrical anticline can be
calculated, as follows : —
I = d tan 0 + X
where I is the distance from the crest to the well, d is the depth
of the well, 0 is the angle of hade of a plane through the apex
of the fold, and X is the distance between what he calls the
" apex- locus " and the " crest-locus." Thus a well at A (Fig.
11) will just touch oil in the petroliferous bed 1, while a well at
D will strike oil in the oilrocks- 3 and 4, but not in the higher
beds 1 and 2. The angle 0 must be found by observation : it
is half the difference between the steepest dips observed on
either side of the crest in the same led. Thus, if the maximum
dip of a bed on one flank is 90°, and on the other flank 10°, 9
will be 90° I 10° = 40°.
136
OIL-FINDING
This formula is of great practical value in determining the
best position for a well, when some evidence is to hand from
the other wells in the vicinity. It leaves, however, several
details to be worked out practically. Thus, unless the depth d
to be drilled is known approximately, it is impossible to find a
FIG. 11. — Section of Asymmetrical Anticline. Dotted portic
extent of oil-impregnation.
shows
value for /. Again, the distance X, which will also vary accord-
ing to the depth, must be found by calculation ; it depends on
the shape and sharpness of the flexure, which may vary greatly
in different localities. It should be possible to calculate X from
observation, when the strata are well exposed. It will almost
certainly decrease gradually as lower and lower horizons are
reached.
But the calculation of d is a matter of greater difficulty,
unless evidence from other wells is available, or the oil-bearing
horizons are known through very careful stratigraphical work.
Thus in a new field that is to be tested it will be expedient to
place the test-well so that the crest will not be crossed at the
greatest depth to which it is proposed to drill. This may
necessitate the missing of the oilpools at shallow depths, so
LOCATION OF WELLS
137
the geologist must consider each case on its merits and locate
his well for deep or shallow oilrocks as is most convenient or
most likely to prove of value to the company developing the
area. As a general rule, it will be found better to exploit the
shallow sands first, once the presence of oil has been proved,
stating the depth to which each well is to be drilled, while
another well can be located further from the crest to test deep
oilrocks.
One point with regard to asymmetrical anticlines and domes
seems to have been lost sight of very frequently, and that is,
FIG. 12.— Asymmetrical Anticline, showing decrease in Hade of Axis.
Dotted portion shows extent of oil-impregnation.
that the hade of the axial plane in any flexure is not constant.
Tracing the axis along the flexure, the hade is seen to decrease
or increase, but it is not so readily admitted that traced down-
wards into the heart of the fold the hade must decrease. Yet
when we consider that the flexure has been caused by tangen-
tial stress, it is obvious that the hade of any asymmetrical
flexure must decrease downwards, and finally disappear alto-
gether. In Fig. 12, which represents an asymmetrical fold on
138 OIL-FINDING
a scale sufficiently small to include practically the whole of
the flexure, it will be seen that the hade of the axial plane
decreases rapidly, and that to calculate on its remaining con-
stant and to drill on that theory would be to court failure.
Again, it will be seen that at a certain distance from the crest
the hade beneath the surface has practically died out ; no well
need therefore be drilled further from the crest than this point,
the approximate position of which can usually be arrived at by
a careful study of sections taken across the whole flexure. No
formula can be given for finding the distance of this point from
the crest, but as it will be a suitable place for all wells greater
than a certain depth, it should be the geologist's endeavour to
ascertain the position of the point as accurately as possible and
to locate the deep test of the field upon the line indicated.
Another point is illustrated in the section. It will be seen
that in a well which has been placed too far from the crest, a
thin bed of porous rock has been pierced almost on a level with
a thicker oil-bearing band which forms the crest of the fold.
It is not an unfrequent phenomenon to encounter a show of gas
and filtered oil in a thin and probably lenticular band placed
in such a position. The oil which it contains has come from
one of the main oilrocks, and has been filtered during its migra-
tion. It is struck at a depth much less than that calculated
for the main oilrock that the well has been drilled to strike.
The occurrence of this show of filtered oil is very naturally
regarded as a hopeful indication, and the well may be continued
to a great depth, though owing to the decrease of the hade of
the axial plane it is impossible to strike oil in the main oilrock.
Instances of this have come under the writer's knowledge more
than once, and till the whole underground structure of the field
has been clearly proved, it may be impossible to satisfy oneself,
or those responsible for the exploitation of the field, that the
show of filtered oil need not be a hopeful indication at all.
These considerations apply chiefly to highly asymmetrical
anticlines, where the flexuring is sharp. In flexures that are
only slightly asymmetrical and are not very sharp, it matters
very little on which side of the crest the well is drilled. It is
possible to obtain oil in quantity from the steeply- dipping flank
of an anticline, but it is obvious that the reservoir on that side
can never be so large as on the more gently-dipping flank.
The migration of petroleum towards the crest is also a simpler
LOCATION OF WELLS 139
matter in steeply-dipping beds, and a well-defined water level
may quite possibly be found on the steeper flank considerably
above the water level on the gentler flank. This is due to the
hydrostatic pressure of water, which, according to theory,
probably underlies the oil ; in steeply-dipping beds the separa-
tion of oil and water by migratory movements along or up the
bedding planes is naturally favoured more than in gently-
inclined beds. Accordingly, it is always as well to make
locations on the gently-clipping flanks of anticlines whenever
they show asymmetry. Another reason of a practical kind
emphasizes this desirability ; the mechanical difficulties of dril-
ling through steeply-dipping strata are, in most cases, much
greater than when the strata are gently inclined, and the
tendency of the bore to depart from the vertical position, and
for the sides of the borehole to cave, are always greater, the
more steeply the strata are dipping.
The asymmetry of a fold very frequently changes when it is
traced some distance, and the actual hade of the axial plane
may change from one side to the other ; hence in every locality
the amount and direction of hade must be ascertained as care-
fully as possible and locations made according to the cir cum-
stances in each case. Where there is any doubt as to the
position of the crest at any particular depth, to make sure of
reaching the oil horizon on the gently-dipping flank rather than
on the steep flank should be the geologist's endeavour. Except
in long anticlines with little or no sign of dome structure, and
where the oilpool is consequently very narrow, there should be
little danger of missing oil if the effects of hade are carefully
worked out.
When flexures are intensely folded, or even overfolded, as
in some cases in Galicia where a single oil-bearing stratum has
been pierced three times at different depths in the same well,
the conditions are apt to become so complicated that it is
impossible to state any general proposition that will serve as
a guide in locating wells so as to give the best yield. But it
is as well to remember that a water-level will be found some-
where in almost every oil-bearing rock, however well isolated
by surrounding impervious beds. The geologist, in estimating
the area from which a production of petroleum is probable, and
the area likely to be drained by a well, must go by such evidence
as is available either from other wells in the same area or from
140 OIL-FINDING
productive wells in other areas, assuming that an oil- water-level
will be discovered, and leaving the local variations in this hypo-
thetical plane between water and oil to be proved by actual
drilling. Local variations due to differences in porosity, split-
ting, thinning out, or lenticularity of porous beds, and seepage
across fault-planes are very common, but cannot be reckoned
upon till proved by the evidence from a number of bores.
In locating wells to prove the extent of a field in which oil
has already been struck, the geologist must use his common
sense when guiding evidence is deficient. Thus if an anticline
exhibits dome structure, that is to say, if well- defined pitches
point to the length of the field not being excessive in com-
parison with its breadth, the oil reservoir in each productive
stratum will be deep, and it may be possible to locate profitable
wells far down the flanks or pitches of the flexure, while if the
fold be little affected by pitches a shallow, long, and narrow oil
reservoir may be expected. In any case the geologist will find
it expedient to feel the way cautiously towards the limits of
an oilpool, rather than to locate wells rashly in the hope of
proving a wide field at once. It is hardly ever profitable to
drill an unsuccessful well, as the evidence it furnishes is almost
entirely negative, and does not necessarily assist those in charge
of drilling operations in defining the limits within which profit-
able productions can be obtained. On the other hand when
water and oil are found in the same stratum when pierced by
a well, when the occurrence of the two liquids can be demon-
strated in intimate association, very valuable evidence as to the
extent of the oilpool may be furnished. As stated above, it
is useful and even necessary to assume that there is a regular
level between oil and water in each bed, a horizontal plane
above which oil, and below which water, will be struck, but in
actual practice, especially where the oil is of high specific
gravity, it may be exceedingly difficult to determine where
such a plane can be drawn in horizontal sections. In simple
and well-defined structures, where the porosity of the oilrocks
is fairly constant and the oil of light gravity, there may be
little difficulty, but even in such a case the plane may be at
different levels on opposite sides of an anticline. It is a useful
convention, but it must not be regarded as a hard and fast line
which cannot be affected or altered by local conditions. There
are many cases on record of water being struck in a well and
LOCATION OF WELLS 141
pumped for months before oil has made its appearance in any
appreciable quantity, and yet the well has finally yielded oil
without any admixture of water and continued to give a profit-
able production for years. Thus the actual striking of water
where oil is expected does not always mean that the well is a
failure. Again, what is called a "freak well" — a deplorable
phrase — may be brought in outside what has previously been
accepted as the limits of profitable drilling. Of such freaks
there is always an explanation, though it may be by no means
obvious ; in many cases such so-called freaks could have been
foretold, had the geological conditions been studied with suffi-
cient care.
Many of the discrepancies between predictions and results
nowadays are attributed to lenticularity of the oil-bearing
strata. Oilsands are doubtless lenticular, as deltaic and
estuarine deposits must necessarily be, and as for that matter
every clastic deposit in the world must be. Among the rapidly
deposited sediments of a delta thinnings out and variations are
naturally especially conspicuous, but, all things considered, the
lenticularity of oilsands is being made too much of. To shelter
oneself behind " that comfortable word " lenticularity when
predictions as to the depth and position of oil-bearing strata,
or the prospects of a well, have gone astray is a confession of
weakness, ignorance, or, still more probably the want of careful
detailed mapping, which the geologist should be ashamed to
make unless he is in a position to prove out and out that such
lenticularity exists. As a general rule,:if he can prove striking
lenticularity in the beds exposed at the surface he may be
justified in assuming it among beds of similar character and
mode of formation underground. In any case he should be
able to ascertain the general directions of lateral variations,
and should thus have the key to any problem involving the
sudden thinning out of beds of porous strata capable of con-
taining petroleum, or the sudden appearance of such strata.
To depend upon well-records for such evidence is at the best to
obtain information at second-hand, and it is not in every field
that well-records can be implicitly relied upon, the personal
equation entering into them to such an extent that, even where
carefully kept, they may leave many essential points doubtful.
To advocate drilling down the pitch or the flank of a flexure in
the hope of striking a lenticular bed impregnated with oil and
142 OIL-FINDING
sealed from the invasion of the dispossessing fluid, water, by
being surrounded by impervious strata, is to reduce geological
science to the level of guess-work. Yet wells have been suc-
cessfully brought in under such conditions and have proved
very remunerative, though the locations have been disapproved
of by geologists on grounds perfectly justifiable. It is such
instances that have often discredited geological work in the
minds of practical and unscientific oilmen, and it makes the
geologist's task all the more arduous to know that unexpected
and even unprecedented conditions may falsify the conclusions
at which he has arrived after the most careful consideration of
structural and practical evidence from every point of view.
It is for this reason that the study of lateral variations has
been insisted upon with such emphasis ; oilsands can be shown
to be splitting up, thinning and dying out by evidence visible
at the surface, as the directions of such splitting, thinning, and
dying out can be ascertained beyond question; is it unjusti-
fiable to assume that similar variations must exist beneath the
surface, and that from what can be actually seen we may
interpret the subterranean anomalies of which we only obtain
direct evidence through the drilling of wells ? Lenticularity of
beds may be a very important factor in oilfield work, but to
assume it as an explanation of facts that have not been antici-
pated may be merely a begging of the question. In locating
wells upon an anticline, especially if it be of considerable
extent and length, all these matters must be considered, and
it is rash to assume that an oil horizon proved at one end of
an anticline must necessarily persist to the other end, even
where the structure is eminently favourable for a production of
petroleum.
In locating wells upon a monocline or a terrace- structure,
the geologist has, as a rule, a very simple task. He will be
guided first of all by any local variations of dip or strike that
may be observed, and secondly by the presence of surface
indications. Where the dip decreases locally, or where there
is a sudden change of strike, especially if the bend in the
strike is concave towards the direction of dip, the locality
will generally have better prospects of production than areas
lying to either side. In a terrace-structure, where the oil-
bearing strata do not crop out at surface, this has been proved
in many instances ; in monoclines attention is usually called to
LOCATION OF WELLS 143
such favourable localities by the " shows " at outcrop, for it is
at such changes of dip or strike that the petroleum tends to be
concentrated and frequently appears at the surface.
It only remains to calculate at what depth it will be advis-
able to strike the oil-bearing rocks, to measure off a sufficient
distance in the direction of dip, and mark the location. On
terrace-structures it may not be possible to calculate the depth,
and the procedure will be as in the case of gentle anticlines
with a slight degree of asymmetry.
When locating on a monocline it may be taken for granted
that a water-level will be found somewhere, though it is
possible that both oil and water may be encountered together
throughout a considerable thickness of strata. This depends
largely upon the specific gravity of the oil, and, as by gradual
inspissation at and near the surface the oil in an outcropping
petroliferous band must, however slowly, lose its lighter
constituents and become heavier, a final stage may be reached
when the oil approximates in specific gravity so nearly to
that of water that replacement by the latter cannot be complete ;
consequently a definite water-level, even if proved in one
locality, may not be constant over any considerable distance
in the outcropping oilrock. But it is as well to assume that
a water-level will be reached sooner or later, and, therefore,
the oilrock must not be struck at too great a depth. At too
shallow a depth gas-pressure may not be great enough to
ensure a good production, and the oil may be too much affected
by inspissation. It follows that the making of a location
requires the exercise of judgment and will be governed chiefly
by experience of results obtained in similar strata and structures
and with similar oils. Localities where the dip is lowest will
be selected in preference to those where the inclination of the
strata is considerable for several reasons; in the first place
because it is then possible to place the well further from out-
crop for a given depth, and secondly because seepages at
outcrop may not have depleted the petroliferous bands to such
an extent. A depth of 400 feet is very suitable for a first
well when the strata are inclined at an angle of 20 degrees or
less. This gives a minimum distance from outcrop of between
1100 and 1200 feet. If the strata dip very gently, the depth
need not be so great in a first test. After one successful well
has been drilled, the next can be placed to strike the oilrock
144 OIL-FINDING
at greater depth, and the limits of the area which will prove
profitable to drill felt for cautiously.
With beds dipping at 45 degrees or more, 600 feet will
not be too great a depth for the first test- well. In the case
of light paraffin oils, as has been explained before, such tests
may be quite unsuccessful, but with oils of asphaltic base
excellent results may be obtained under such conditions.
The calculation of depth is a matter of great importance,
especially as the shutting off of any water-sands that may be
found above the oilrocks is absolutely essential if good results
are to be obtained. Given a careful geological survey of the
area there should be no difficulty in calculating the position
of the oilrocks and the water-sands beneath the surface at any
point, and it may be possible even to draw contour lines
showing the approximate depths. But the field-student must
be warned against projecting the angles of dip as observed at
the surface and so attempting to delineate the underground
structure. Such methods as those used by the mining engineer
in calculating at what depth a shaft must be sunk in any
locality to strike a lode will, if applied to oilfield work, often
1 give results so inaccurate as to be useless for practical purposes.
It must be remembered that any monocline or any inclined
bed represents part of the great curve of an earthwave, and
I that the part seen at the surface is infinitesimal compared
with the part concealed beneath, so that the angle of dip,
however carefully measured, may not be very useful as a
! guide. The drawing of horizontal sections to scale, when there
is sufficient evidence, will make this obvious at once, and will
1 emphasize the futility of projecting a dip as seen at surface,
as if it continued indefinitely without increase or decrease.
It is expedient, therefore, to make a careful horizontal section
before attempting to make locations, provided, of course, that
the section is made to scale from a geological map, and is not
merely the diagrammatic absurdity produced by an observer
who has made no serious attempt to map the ground geologically.
Thus we come back to the proposition stated above that the
location of wells should depend entirely on the geological
mapping, and provided that this has been done with reasonable
care there can be little doubt as to where a test-well should
be placed.
It would serve no useful purpose to take every kind of
LOCATION OF WELl!S
H5
geological structure, and give in detail an account of the
conditions which should determine the site for a well : in spite
of elaborate classifications of structure, all structures known
in an oilfield can be considered under two or three compre-
hensive heads. But a few words are necessary about areas where
faults are a conspicuous feature. Great care must be exercised
in locating wells in faulted areas, not only because the fault
plane if pierced during the drilling may be the cause of great
mechanical difficulties, making the keeping of the bore vertical
and the sides from caving by no means an easy task, but
because the presence of faults in the near neighbourhood may
FIG. 13. — Diagram showing how a small fault may enable a well to tap
a great thickness of oilrock locally. Arrows show movement of oil
and gas.
have great effects upon the production of the well. The theory
that faults affect a field adversely by allowing migration of oil
along the fault- plane has already been dealt with and disposed
of, but by allowing communication between separate oilsands
across the fault-plane a dislocation of the strata may have
remarkable results (Fig. 13). The field-work of several
observers has proved that many of the greatest fountains in
the Baku field lie close to the line of a fault, which has made
possible communication between separate oilsands, which are
both thick and numerous, so that a well on or near the line
of fault is able to derive oil from many horizons, and to tap
L
146 OIL-FINDING
them, so to speak, all at once. A somewhat similar ease can
be cited from the Yenangyoung field in Burma, where Mr.
B. F. N. Macrorie, of the Burmah Oil Company's Geological
Staff, has shown how small faults of little structural importance
have assisted in raising the production of certain wells far
above the average, and limiting the productiveness and life
of others.
As a general rule it may be taken that it is always
preferable to drill on the upthrow side of a fault rather than
on the downthrow side. The reasons for this are easily under-
stood when it is remembered that any fault can be theoretically
replaced by a sharp fold; on the downthrow side the throw
of the fault may be sufficient to bring the horizon of an oil-
bearing band below water-level, while with normal faults,
hading to the downthrow side, the well may encounter the
fault plane and get into considerable mechanical difficulties.
In many cases what is seen as a fault at the surface becomes
a sharp fold when traced downwards where the elasticity
of the beds is greater, especially when thick and soft
masses of argillaceous rock are present. Faulting when it
occurs in a series where by far the greater part of the strata
is impervious, and the porous oilrocks widely separated, may
be of great importance, as an oil-bearing band that would other-
wise be found cropping out at the surface may be cut off and
isolated among the impervious strata. The oil contents may
be preserved thus from inspissation and great productions
may be obtained from a band isolated in this manner. It will
be observed that the throw of the fault may not be a matter
of importance in this case ; either an upthrow or a downthrow
may effect the isolation. It is obvious that careful geological
work is necessary before it is possible to locate wells to take
advantage of structures such as that shown in Fig. 13, but
in many fields unexpectedly large productions have been struck
by the drill entering a band of oilrock which has been pre-
served from weathering and the loss of light oils by being
cut off in a similar manner.
Faults, generally speaking, unless they are dislocations of
great size and throw, are more helpful than harmful in an
oilfield, for the simple reason that in most productive fields
the total thickness of impervious strata is in excess of the
total thickness of porous rocks. Their presence may complicate
LOCATION OF WELLS 147
the geological map and make the calculation of the depth to
be drilled in a well more difficult, but their presence need not
have any deleterious effect upon production.
Questions of accessibility, proximity to water supply,
expenses of road- making, etc., must all be taken into account
when making a location for a test-well in a new field, but all
these matters, though serious items in expenditure accounts,
must be regarded as secondary to finding the site most favour-
able according to the geological conditions. The young
geologist may have pressure brought to bear upon him to fix
upon some alternative location which seems " almost as good "
as the one he had originally selected, or which may perhaps be
in a locality where the prospects of obtaining oil are doubtful,
but which is much more easily accessible and will not
necessitate any great expenditure in road-making, transporta-
tion of plant, and furnishing with a water supply. He will do
well to resist all such suggestions, because it is a short-sighted
policy that advocates a first test-well in any but the most pro-
mising locality available. The cost of drilling a deep test-well
in a new field is usually so greatly in excess of the expenses
incurred in road-making, providing water supply, etc., that these
may be disregarded. If the more accessible site be chosen, and
after months, or, if any difficulties be encountered in the drilling,
perhaps more than a year spent in completing a deep test with-
out successful results, another well costing probably nearly as
much and taking as long to drill will have to be tried before
the area can be considered fairly tested. On the other hand it'
the best site, geologically speaking, be selected at first, and the
test be unsuccessful, the area may be abandoned at once, and
all the time and expense of drilling a second well saved. It
may often be difficult to convince field-managers or managing
directors that an area can be thoroughly tested by the drilling
of one well, but if the geological work has been done thoroughly
one test should be sufficient in almost every case, and when the
first test is unsuccessful the throwing away of time and money
by making further tests is a matter the blame of which must be
largely at the door of the geologist, unless his advice has been
arbitrarily overruled.
Very frequently a geological adviser finds himself in the
position of having to advocate the testing of an area to a certain
depth, and after that depth has been reached without striking
148 OIL-FINDING
oil it may be necessary to say at once, and as definitely and
strongly as possible, that there is no further hope, and that the
area should be abandoned. In such a case, if the well be " in
good shape " to be carried much deeper, there may be consider-
able hesitation on the part of those responsible for the practical
operations in deciding to abandon it. The geologist, having the
courage of his own convictions, should make things as easy for
the field-manager as he can, by putting the case clearly and
concisely before him. Little blame can be attached to the
unsuccessful testing of a new field by drilling one well, as it is
often impossible to make sure of the petroliferous character of
part of a series in any particular locality without evidence from
a borehole ; but to allow a second unsuccessful test to be
drilled, or the first to be continued to a great depth when it has
no further prospect of striking oil, is a confession on the part of
the geologist of the uncertainty of his own judgment or his
ability in reading the evidence obtained during the geological
mapping of the ground. Hence it becomes of the utmost im-
portance that no testing of a new field should be commenced
till the geological examination has been made in detail and the
location made in the best possible place to obtain a production
of oil. Were the importance of this principle more fully
realized, the popular idea of the capricious nature of petroleum
would be shaken, and might even be relegated to the liinbo of
scientific fallacies.
After a successful well in a new field has been drilled, the
second test should be placed so as to develop as large an area as
possible without taking the risk of getting beyond the margin
of the oil-reservoir. This is to enable some idea of the area
available for drilling to be obtained at once. When there is
great doubt as to the extent of a field, the best policy to adopt
in developing it must necessarily be uncertain, but with a fairly
accurate idea of the minimum size of a new field, drilling pro-
grammes and transportation of plant can be taken in hand in
the most economical and adequate manner. The only exception
to this is when the petroleum is required, and can be handled,
at once ; new wells may then be started near the first test.
This, however, is a state of things- by no means usual in new
fields.
Second and third tests should not be located directly down
the dip from the first producing well, but at some distance to
LOCATION OF WELLS 149
the side, so as not to interfere with the supply of oil to the first
well. The natural migration of petroleum will be up the dip-
slopes in most fields, whether in monoclinal or anticlinal
structures. If the first test- well has been placed on one flank
of a symmetrical anticline, the second may be located on the
other flank, in order to obtain information as to the breadth of
the field. In a dome structure the second test should be made
in the direction of the larger axis of the dome.
The distance at which wells may be placed from one another
without mutually affecting their production is a question upon
which it is impossible to dogmatize, as it depends upon so many
factors, such as the porosity of the oilrocks, the grade of the oil,
and the gas-pressure, which may be different for any different
field. It may be taken as an axiom that in any given field
there < is a certain minimum number of wells which will
exploit the area most profitably and economically. To drilj.
more than that minimum number will not ensure the pro-
duction of more oil in the long run, but less, for though pro-
duction may be more rapid, gas-pressure will be dissipated
more quickly, and thus the motive force that brings the
petroleum into the well, and perhaps up to the surface, will be
to some extent wasted. Fields such as Spindle Top, in Texas,
and Twingon, in Burma, might have had very much longer
lives and produced much more oil with a fraction of the expense,
had there been any regulations to prevent over-drilling. '
With light paraffin oils, high gas-pressure and porous sands,
a distance of one hundred yards between wells will probably be
found a convenient and sufficient distance. When the sands
become partly exhausted or clogged near the bottoms of the
wells by the deposit of solid paraffin, new wells may often be
drilled with profit between the old producers. In shallow
fields with asphaltic oils, and in oil-bearing limestones, wells
may be placed considerably closer without seriously affecting
each other, but in each field the requisite minimum distance
must be ascertained by experience.
In calculating the number of producing wells which a
proved area will carry it is advisable to allow for a distance of
from 200 to 300 feet between each.
Though it is no part of the geologist's task to give advice as
to the methods to be made use of in drilling, practical experi-
ence in oilfields will soon make him au fait with the chief
150 OIL-FINDING
mechanical difficulties that the driller will have to overcome,
and it will be part of his duties to acquaint the field-manager
or driller with the nature of the strata through which the well
will penetrate.
This will enable those responsible for the drilling to select
the best methods for overcoming the difficulties which each
kind of rock will present, and the type of rig and tools most
suitable will be chosen. Thus through a thick soft argillaceous
group it may be found most profitable to use a rotary rig, while
drop-drills and under-reamers may suit a variable series con-
taining hard calcareous bands.
The approximate depths of probable water-sands, the
presence of hard bands upon which it will be possible to ground
casing, the occurrence of soft beds liable to cave into the bore-
hole, are all points upon which the geologist may give infor-
mation that will be of great value to the practical driller. Again,
the angle of dip, if it is high, is an important matter, since
steeply inclined beds are frequently liable to cave, and if thin
hard beds are encountered dipping at a'high angle there may be
great difficulty in keeping the bore vertical.
Thus, in return for the information afforded to him by the
log of a well the geologist should be able to forewarn the driller
of difficulties, and so ensure that they are taken in hand and
overcome most expeditiously.
CHAPTER IX
(FOR BEGINNERS)
FIELD WORK
IN the preceding chapters allusions to geological mapping have
necessarily been very frequent, and it is hardly necessary at
this stage to insist that the object of all geological field-work
must be in the end to make as complete a geological map as
possible. No casual examination of an area is sufficient, no
spending of a few hours, or even a few days, if the area be
large, in examination of sections and oilshows and the taking of
notes will qualify the geologist or petroleum expert adequately
to advise those who are undertaking development work. It used
to be one of the distinguishing points between the amateur and
the professional geologist that the former was frequently content
with the drawing of a horizontal section, while the latter
alway pinned his faith to a map, but nowadays the amateur
is learning that in any case the map must be made before the
section, and that nothing but a map will suffice. In oilfield
work the whole concession or area, and frequently a large area
outside of it, must be mapped geologically.
In some cases a published geological map may be available,
and may be of great assistance, but it is not likely to be on a
sufficiently large scale to give the details which are essential, if
wells are to be located with accuracy to strike the oil-bearing
deposits at the determined depth. The best topographical map
available must be procured, and if it be on too small a scale it
may at least serve to check distances and compass-bearings in
the large scale map which the geologist will prepare for himself.
The smallest scale that is at the same time sufficiently large to
admit of mapping in detail will be naturally selected ; for most
fields and field-geologists the scale of six inches to the mile will
be found to meet the case. Eight inches to the mile is also a
very useful scale, and in producing fields scales of sixteen or
152 OIL-FINDING
twenty-four inches to the mile may be used with profit, and
may, indeed, be necessary; but for all practical purposes,
especially in new fields and in wild and unopened country, the
six-inch scale is probably the best.
Experienced geologists will pardon the writer for giving
some account of the methods of field-mapping that he has
found most effective under different conditions, in the hope that
some of them may prove of value to the prospector or field-
student, for whom this little book has been written.
Many of the details to which much attention is given in
large-scale mapping in Britain can be neglected, partly or
wholly, in oilfield work, and on the other hand methods and
conventions that are not required in ordinary geological
mapping may become of the greatest importance when an oil-
field is being surveyed. It is necessary that structure be
worked out thoroughly, and it is for this reason that the
mapping must be done on a scale sufficiently large and in
sufficiently great detail to make any mistake in structure
impossible. But the nature of the strata and the mapping of
outcrops with great accuracy, and determining the exact
positions of points may in many cases become matters of minor
importance. The determination of the exact position of the
crests of sharp anticlines, the angles of hade of the axes of
asymmetrical flexures, and the pitches of axes is essential, and
consequently observations may have to be taken very frequently
and with great care, while the angles of dip on the flanks of a
flexure may not be considered of sufficient importance to
demand any special care in the taking of observations.
Equipment. — The geologist who undertakes the examination
of oilfields must have an effective, but not necessarily an elab-
orate equipment. The first essential is a good and substantial
map-case. The large leather map-case as used by the Geological
Survey of Great Britain is a very good model, though it may be
improved in details to suit the individual. It allows six square
miles of ground on the six-inch scale to be studied at one time,
without changing maps, an ample area for all practical purposes.
It is slung from the shoulder by a strap and can easily be
manipulated with one hand ; this may seem a very trivial point,
but it is really of great practical importance. Smaller map-
cases or mounted and folded maps carried in the hand or in a
bag or pocket will be found troublesome to manipulate, and do
FIELD WORK 153
not conduce to good geological mapping. The tendency will
naturally be not to consult the map frequently enough, and the
mapping may become more of the nature of taking occasional
notes. The possession of a good handy map-case opened and
managed with one hand will do much to teach the field-student
practical mapping and the reading of geological maps.
Plane- tables, though excellent for careful work in small
areas, are too cumbersome : the geologist has very seldom suffi-
cient time at his disposal to make use of such appliances, and
the slight gain in accuracy obtained by using them is more
than counterbalanced by the laborious nature of the work and
the waste of time involved.
Cavalry sketching-boards, fitted with a compass and designed
for use on horseback, are pretty little toys. They may be of
use on a preliminary traverse or a pioneer exploration of new
countries, but they are too small for detailed and accurate
work, while the compass is usually also too small to take bear-
ings with sufficient accuracy. Furthermore, if the possession
of such an equipment has the effect of inducing the young
geologist to imagine that efficient geological work can be done
on horseback, it may be his ruin so far as practical field-work
is concerned.
For instruments, the first essential is a good pocket compass,
one at least two inches in diameter, with a clearly marked dial,
that will enable the observer to take bearings to within two
degrees. This compass should be carried in a case from which
it can be taken and manipulated with one hand. The saving
of time, trouble, and, it may even be added, temper, that is
effected by carrying a compass that does not require two hands
is enormous ; this can be understood when bearings have to be
taken once at least in every fifty yards, as is necessary when
working in dense forest. It is as well to have this compass
combined with a clinometer sufficiently reliable to take angles
of dip without an error of more than one or two degrees.
For taking bearings from distant points a good large
prismatic compass is necessary ; it must be sufficiently sensi-
tive to read correctly to half a degree, but the needle must not
be too " lively." That is to say, though sensitive, the card
should have a comparatively high " moment of inertia." This
will enable readings to be taken by the method of oscillations,
and another great saving of time will be effected. The geologist
154 OIL-FINDING
will soon learn to recognize the happy mean between too great
mobility and too great sluggishness in a prismatic compass.
An Abne'y's level, or some similar instrument, is sometimes
necessary in taking readings of the angles of pitch and dip
where these have to be measured very carefully, but it need
not be carried always. In producing fields and open ground it
is far more likely to be required than in new and unexplored
country.
Theodolites, tachyometers or omnimeters are often of great
value in open ground, especially where there is no topographical
map available, but it is impossible for the geologist to carry
such instruments with him in rough jungle work. The young
geologist should have no ambition to make himself a third-rate
land-surveyor, and though it is necessary to understand the
use of these instruments, and to be able, if it is required, to
measure a base-line with them, he will be well advised to use
them as little as possible ; to give undue attention to the more
or less mechanical duties of land-surveying may run away with
time that may be more usefully employed in geological work.
Like the Abney's level the omnimeter or tachyometer may be
left at headquarters, and only taken out when some special
work with it becomes necessary.
A good protractor adapted to the scale used in mapping
must be procured. This may have to be made specially of
ivory or aluminium according to the taste of the geologist.
Ivory is perhaps the better material, though it warps badly in
hot weather. The six-inch protractor used by the Geological
Survey of Great Britain, and furnished on the back with handy
tables to enable thicknesses of strata, depths and gradients to
be calculated rapidly, is quite the best instrument of the kind
for six-inch mapping.
A hammer may be carried if required, but in Tertiary strata
it will not often be used; a cutlass, machete, dah or kukri
will be as effective, and will serve other useful purposes, e.g. in
clearing a path through thick jungle or in digging down a grass-
grown section to lay bare the strata.
A stout walking-stick with a crooked handle by means of
which it can be hung on the arm when using the map-case or
compass is almost invariably carried by the writer. In taking
the dip of a ripple-marked sandstone it may be laid upon the
the surface of the rock and the clinometer placed upon it. In
FIELD WORK 155
slippery or soft ground or in rock-climbing it may also be very
useful, and in tropical countries where snakes are numerous it
may be necessary as a weapon. The carrying "of a stick is,
however, a matter upon which the individual must decide
according to his inclination.
Pencils, hard or soft, will be chosen to suit the material
upon which mapping is done, and the climate, whether wet
or dry. A few coloured pencils will be found of great use, and
they should be carried so that the colour of each can be seen,
and any one selected and brought into use with one hand. A
good india-rubber is of course essential.
As to the material on which the mapping is to be done, the
author, after trying many varieties from tracing linen to What-
man's boards, has come to the conclusion that oiled paper
mounted on linen combines the greatest number of advantages
with the fewest defects; it does not shrink or stretch appreciably,
it is not rendered useless by damp, takes pencil and chalk
marks clearly, and keeps a good surface even after much rough
usagp. It is advisable to have the paper cut accurately to fit
the map-case. Thus for the ordinary six-inch map-case the
mapping-paper should be cut in rectangles of twelve by nine
inches.
Some observers favour squared paper for field work, but
if it is really to be of use it must be adapted to the scale on
which the mapping is done. It must tend also to make field
work too mechanical, and does not teach the field-student to
train and depend upon his eye.
A note book is often useful, but is not absolutely neces-
sary ; all notes of importance must be put upon the field-map.
Descriptive notes, lists of compass-bearings, or fossils collected
from various horizons, and small details of mapping or sections
shown on a larger scale than that employed on the map can be
kept in note books, but as a rule all these can be put in con-
densed form on the field map.
Finally a strong water-proof bag or satchel, capable of
holding the map-case during rainstorms, and with an extra
pocket for other instruments, is an essential part of the
geologist's equipment. Willesden canvas is a very suitable
material for such a bag, especially when bound with leather
and slung on a strong leather strap for an attendant to
carry.
156 OIL-FINDING
The geologist will do well to carry all the instruments
he is constantly using himself. Hammer, Abney's level, and
occasionally cutlass and prismatic compass may be carried by
one of his attendants, but everything else should be disposed
about his person in such a manner that it can be brought into
use with the least delay and fumbling. It may be thought
that these are trivial details, the neglect of which can be of no
possible consequence ; but if the field-student has to work in
the tropics in a temperature of 100 degrees Fahr. or more in the
shade and 160° or 170° Fahr. in the sun, he will find that even
trifling details become of importance, and trifling annoyances
may be magnified into trials. To have to wait while a lazy
native servant comes up with the instrument required, and
slowly unloads a bag in search of it, to have to hunt for a
coloured pencil among several concealed in a pocket, when the
required one is always the last to appear, and to repeat these
performances fifty or a hundred times a day is enough to
become a serious worry to the geologist struggling with
climatic conditions to which he is not accustomed, and his
work may really deteriorate and become less careful through
lack of attention to such details. Again, the time occupied
in the making of a geological survey is often a matter of great
importance. Kival geologists may be in the field, other interests
may be represented by other prospectors, and it may depend
largely upon the speed with which the main points of a struc-
ture are elucidated that the success or failure of the company
or syndicate for whom the geologist is acting will turn. Every-
thing, therefore, that favours rapidity in field work, without
decreasing efficiency, is to be cultivated.
Armed with the equipment set forth above, the geologist
may go anywhere and map any ground in the world, provided,
and on this the success or failure of his work depends, that he
adapts his methods of survey to the particular variety of ground
with which he is dealing. The dense forests of Central or
South America cannot be attacked in the same manner as the
barren hills and plains of India or Persia.
It is presumed that the aspirant to become a petroleum-
geologist has had some training in geological mapping on a
large scale before he is called upon to attempt the survey of
a new territory, and if he has had experience of mapping in
Britain on the splendid six-inch maps of the Ordnance Survey,
FIELD WORK 157
he will start with a great advantage over others who have not
been so fortunate. The areas which he will have to survey in
new countries where the oilfields of the future are waiting for
development, have in all probability never been mapped topo-
graphically, and he will have to start with blank paper and
construct his own map. In such cases everything will depend
upon the methods by which the survey is conducted.
Survey in Open Ground. — If the ground be open and largely
bare of vegetation, the matter is fairly simple. A base-line,
or still better, two base-lines meeting at an angle, must be
measured and marked clearly on low and, if possible, level
ground, where their extremities can be viewed from the sur-
rounding country. Triangulation by prismatic compass from
and to the extremities of these base-lines will give a sufficient
number of points to form a skeleton upon which to construct
a geological map. Of course such a method is not, and cannot
be, entirely accurate, as the readings of compass bearings with
a hand prismatic compass cannot be vouched for within less
than 30', but a map can easily be constructed by means of
numerous readings and check readings that will be quite
accurate enough to ensure that no error in geological struc-
ture is possible. Should the area prove eventually to be a
productive field, careful land-surveying will have to be under-
taken sooner or later, and topographical maps accurate in
all details constructed, but that is not a matter for the
geologist.
The length of the original base-lines will depend upon
the size of the area to be mapped, and the nature of the
ground; a quarter of a mile will usually be sufficient. The
distance must be measured carefully by chaining, or, if such
instruments be available, by means of a tachyometer or omni-
meter. An alluvial plain, if the area contains such, is naturally
the best place for such measurements. The angle between
two base-lines must be read very carefully by means of an
omnimeter or by prismatic compass. The positions of pro-
minent features, hill-tops, isolated rocks, or trees, conspicuous
bends in the courses of streams, etc., in the immediate neigh-
bourhood are determined by taking bearings from the extremities
of the base-lines, and thus a series of points is obtained from
which secondary points of importance can be fixed upon and
marked on the map. As many check readings as possible
158 OIL-FINDING
should be taken in determining new points, and where dis-
crepancies occur, and the triangle of error is large, the readings
which are most nearly at right angles to each other must be
taken as the most reliable. The top of the paper upon which
the mapping is done should always be taken as true north,
and the magnetic variation allowed for in plotting the results
of the observations made: if the variation be to the eastward,
it is added to the readings of the prismatic compass, and if to
the westward, subtracted. Many square miles can be mapped
by this method, beginning with base-lines of not more than
a quarter of a mile in length, and the resulting map should be
sufficiently accurate to make the working out of geological
structure, and the location of wells to test the area matters of
practical certainty.
Topographical details, except in the case of important cliff
or river sections which must be mapped carefully, can be
sketched in as the geological work proceeds, and must be
regarded as of secondary importance to the purely geological
mapping.
Once the skeleton of the map is prepared, the mapping in
fairly open ground will not be a matter of difficulty, as there
will always be some point visible from which bearings can be
taken. The principal section across the general strike of the
strata, preferably a cliff, river, or road-section, will be mapped
in detail in order that the natural subdivisions into which the
strata range themselves may be ascertained, and prominent
groups of beds differentiated and selected for following out
through the area. Upon the selection of such groups a great
deal depends, especially where variations are frequent and rapid ;
unless such main divisions of the geological series can be
determined, the construction of an efficient geological map is
impossible. To cover an area with innumerable observations
of dip and strike, however carefully taken and noted on the
map, is not geological mapping in any sense of the word, and
may be a mere waste of time since both strike and dip faults,
unconformabilities and lateral variations may never be detected
by such an amateur method, and even pitches and dome
structures may not be recognized if the ground be rough and
much cut up by valleys.
Frequently the strata group themselves naturally, and the
geological boundary lines to be followed are obvious, but in
FIELD WORK 159
very many cases the geological series consists of rapid alterna-
tions of two or three types of strata repeated over and over
again, and it becomes necessary to select a few well-marked
beds neither too near nor too far from each other and to map
their outcrops as far as possible. It may be necessary to map
the outcrops of many beds before one is discovered that persists
and maintains its characteristics over a sufficient area; a
prominent sandstone or limestone bed may thin, split up, and
die out, and it may be necessary to cross to a lower or higher
horizon and carry on the mapping of another band, which,
though not so conspicuous where first observed, extends further
and remains recognizable over a greater area than the bed first
selected. It is better to map a thick bed or small group of
beds than a thin bed, on account of the rapid changes due to
lateral variation.
Where dips are steep it is not necessary to map separately
horizons near to each other, as the structure will be made clear
by the tracing of horizons from 500 to 1000 feet apart; but in
areas where the strata are gently inclined and outcrops con-
sequently become complicated and irregular, horizons separated
by no jnore than 150 to 200 feet should be mapped. In an
area with low dips towards the centre, and steep dips towards
the margins, thin groups will be mapped in the central part, and
the groups differentiated may be thicker and thicker in the
outermost portions.
It is not sufficient to map a number of sections across the
strike and join up the outcrops of the groups as observed, unless
the ground is sufficiently bare to allow the outcrops to be seen all
the way between each dip section. The selected beds or groups
must be followed and mapped to detect any faults, changes of
dip or strike, unconformabilities, or lateral variations. This
method is, of course, somewhat more arduous, and takes up
more time than sketching outcrops between the mapped sections
in which the various groups have been identified, but it gives
absolutely certain and indubitable results and brings out
evidence which might be missed by making use of any less
careful method. Coloured pencils will be found most useful
in distinguishing the horizons followed on the field maps; in
the finished map the areas between mapped horizons form the
separate groups, which will be differentiated by well-contrasted
colours to bring out the structure so that it can be understood
160 OIL-FINDING
at a glance. It is then of very little moment whether or no
the various groups are of the same types of sediment or not,
so long as they are separated by mapped horizons and are
distinctly coloured.
In open and bare ground as in Egypt, Persia, Baluchistan,
and parts of India and Burma, there is seldom much difficulty
in selecting groups for mapping and differentiation, but when
vegetation is thick or the ground obscure the geologist may
have considerable trouble in subdividing the part of the series
that he is dealing with into such groups as will by their out-
crops bring out the geological structure most clearly ; it is in
easy and open ground that the experience is gained that will
enable the field-student to deal effectively with more obscure
areas.
Eye Training. — One point is of the greatest importance to
the young geologist who is undertaking the survey of new
territory. He must train his eyes and learn to be as much as
possible independent of his instruments. In bare and open
ground, where one's position can always be ascertained
accurately by taking cross-bearings upon known points, the
tendency is naturally to rely upon such observations, with the
result that when one is suddenly confronted with a densely-
forested area, one may despair of ever making an accurate
geological map of it, and may content oneself with the observa-
tion of a few dips and outcrops, the result being that a geologist
of better training has eventually to go over the ground
independently and do it all over again.
To begin with, the geologist must learn the scale upon
which he is mapping, that is to say, he must become so familiar
with it that he can judge a distance as seen on the ground
before him and mark that distance upon his map, without
pausing to consider how many yards or feet it is. To pace or
chain a distance and then measure it off on the map by means
of a protractor is no doubt often very useful, but there is no
reason why the geologist should not train his eye by estimating
the distance before he measures it ; to be able to map any
distance up to three hundred yards or a quarter of a mile
without making any measurement is a very valuable asset to
the field geologist. It is doubtful whether any one is so
favoured by nature as to have a special gift for the estimation
of distances, but the faculty can easily be acquired by constant
FIELD WORK 161
practice, and distances up to half a mile have been mapped in
the author's experience with errors of not more than thirty
feet. It is better, however, not to attempt to map distances
of more than a quarter of a mile without some checking
observations. It must be remembered always that estimates of
distance are apt to vary greatly according to the light. The
length of a coast-section with the tropical sun beating upon it
is liable to be underestimated, while the length of a shaded
road-section overhung by trees or a distance in jungly ground
may easily be overestimated. Consequently the field-student
should be constantly practising the transference of a distance
as seen to his map under every condition of light or shade,
afterwards pacing or chaining it and correcting any error he
has made.
The next point in the training of the eye is learning to
transfer observed angles to the map without the aid of a pro*
tractor, and with a very small margin of error, so that when
bearings are taken with the pocket compass the observed
angle can be sketched at once. This faculty can be acquired
very quickly with a little practice; angles of 45 degrees,
30 degrees, and 60 degrees are, of course, very easily drawn,
and the eye soon becomes efficient in estimating smaller, greater,
or intermediate angles quickly. The error should not be more
than 2 degrees, and provided that bearings are not taken from
points more than a quarter of a mile distant, the map will not
suffer in accuracy. When bearings are taken by prismatic
compass the protractor must always be used and the angle
laid off as carefully as possible, but in all ordinary field map-
ping with a compass by means of which the observer can read
a bearing within 2 degrees, and with an eye practised in the
estimation of angles on the map to within 2 degrees, mapping
can be done at a rapid rate, and with wonderful accuracy, pro-
vided that each observation only includes a short distance.
For practical purposes the distance should never exceed a
quarter of a mile.
Having cultivated the faculty of estimating angles and
distances with fair accuracy, the geologist will be able to
make traverses with pocket compass, starting from a known
point, and if possible finishing also at a known point. Such
work, it may be objected, can never be entirely accurate, but
it must be remembered that it is absolute certainty as to the
M
162
OIL-FINDING
geological structure that is to be aimed at rather than meticu-
lous attention to details of topography. A traverse by means
of pocket compass of a mile or a mile and a half in length
should not terminate with an error of more than forty or fifty
yards. Bearings should be taken when possible by prismatic
compass at distances of not more than half a mile; this will
prevent any error from being made. If, however, no check
readings are possible till the end of the traverse, there will
nearly always be an error to correct. This should not be done
at once, but a " correction mark " put upon the field-map
(Fig. 14), and a new start made from the correct position as
determined by compass bearings. Afterwards, when the maps
• ^ \
FIG. 14. — Sketch mapping in the field. C. Start of traverse ; A. Finish of
traverse ; B. Point actually reached. =^ Correction mark. Traverse
starts again from B.
are being inked in, which should be done every day, the error
can be corrected. If the traverse has been very faulty, it will
do no harm to make a second traverse starting from the other
end ; it is better to learn thoroughly the scale on which one is
mapping than to depend entirely on one's instruments. It is
not recommended that pocket compass traverses should be
carried to a distance greater than three miles, and the beginner
may find even that distance too long. In fairly open ground
it will never be necessary to traverse any such distance with-
out being able to check one's position by taking bearings from
some point fixed by triangulation, but in forest land a traverse
FIELD WORK 163
without check of three miles or more may frequently be neces-
sary. It is in bare or comparatively open ground that the field-
student must teach himself to map with that accuracy which
will be his only support when he has to deal with dense jungle,
where no check readings are possible, and where the man who
depends on his instruments rather than on his eye may feel
quite unable to construct a geological map.
Another, but less important, faculty that should be culti-
vated is the estimation of the dip of strata without making any
observation of it with a clinometer. This is a more difficult
matter than the estimation of distance. When it is possible
to look at a bed along the strike the matter is simple, and
every geologist should be able to read the angle within
2 degrees, but it is often impossible to get such a view of
the strata, and perspective views of dip are very deceptive.
Constant practice, however, will enable the geologist to esti-
mate dips very quickly and accurately, but it is not a method
to be used constantly without checks. Whenever it is possible
to take with a clinometer an observation of dip that represents
approximately the true inclination of the beds, and this does
not happen so frequently as the text-books would suggest,
the instrument should be used, but at the same time the eye
may be trained by estimating the angle before observation is
taken.
Where dips are really of great importance, as in producing
fields, or when a series of observations has to be made to enable
a horizontal section to be drawn and the depth at any point to
an oil-bearing horizon calculated, readings with an Abney's level
or some similar instrument up or down dip slopes is by far the
most reliable method. Strange as it may seem it is in this
simple operation of observing a dip, probably the first thing
in field work that the budding geologist learns, that most
mistakes are made, and mistakes that may have very serious
results. The tendency is always (and this applies to the ex-
perienced geologist as well as to the beginner) to exaggerate
the angle of dip. Where bedding planes are not well-bared or
exposed, it is almost invariably that dipping most steeply that
is selected as offering the best surface, and unless a number of
observations in the immediate neighbourhood be taken and
iveraged, the general inclination of the strata will be put at
two\high a figure in degrees.
1 64 OIL-FINDING
Again, and this is especially true of Tertiary deltaic and
littoral deposits, the dip of bedding planes may be at a very
different angle from the general inclination of the series. Even
where no false bedding can be detected, the strata probably
have not been deposited in a horizontal position. Theoretically
in fact, deltaic deposits are not deposited horizontally, and
amidst the rapidly varying and quickly accumulated deposits of
a Tertiary delta, where much of the petroleum-geologist's work
will be done, it is by no means easy to make sure of the
average inclination. Where folding is well marked, dips may
change every few yards, and not by regular gradations, but
often suddenly, so that quite apart from irregularities of original
bedding the determination of the true dip at any point may be
a very difficult matter. The only method in such cases is to
make many observations on all sides, if there be sufficient
evidence, and to take an average both as regards strike and
dip, always remembering that the minimum inclination ob-
served is more likely to be correct than the inaxinmra. Strike
is in any case more important than dip, and it is always as well
to mark the strike of a bed, even when it is impossible to
ascertain its true dip. It is because dips have to be averaged,
and because it is inadvisable to give too much weight to a few
isolated observations of the inclination of strata, that the
method of estimating dips by the eye alone is frequently suffi-
cient for all practical purposes.
In recording on the map an observation of dip, the point of
the arrow should be marked as nearly as possible on the spot
where the observation was taken.
Surveying in Jungles or Forest Land. — Lack of evidence is
always the greatest difficulty in the way of making an intelligible
and accurate map, and it is in the making of an intelligible and
accurate map where evidence is meagre, that the experienced
geological surveyor proves his ability. Any one can map strata
that he can see exposed, but where exposures are few, or perhaps
entirely wanting over miles of country, new methods have to
be devised, new kinds of evidence have to be studied, and
nothing may be too small and insignificant to give some hint as
to the strike or dip of concealed strata. Unless evidence be
studied minutely in more or less open ground — such matters,
for instance, as the colour and texture of soils, the vegetation
that grows on different varieties of deposit, clay, sandstone,
FIELD WORK 165
limestone, as the case may be, outcrops of water or petroleum —
the key to the structure of obscure or wooded country may
be lost.
One frequently sees it stated that there are " indications of
petroleum " in a certain district, but that " it is impossible to
ascertain the geological structure, as the ground is too densely
clothed with vegetation." In other words, the geologist has
been unable either from want of time, want of sufficient care,
or the lack of reliable methods of surveying, to determine the
geological structure. With the single exception of alluviaj.
flats so vast in extent that the particular area, the geological
structure of which is in question, is too far from any of the
margins where reliable evidence can be obtained, there is no
part of the world's land surface where such an impossibility
exists. An ice-sheet may be considered as an exception to
this, but it is hardly to be regarded as a land-surface.
Alluvium acts as a sponge, wiping out all direct evidence,
though where belts of alluvium are not very large, their very
presence may furnish valuable negative evidence ; but no other
covering, whether of glacial drift, blown sand, peat, vegetation,
or coral terrace, is sufficient to prevent some details of geological
structure being found somewhere. It is with the dense vegeta-
tion difficulty that the petroleum-geologist has to deal in many
parts of the world. Tropical forests, such as those of South
and Central America, or the bamboo jungles of India, are
perhaps the most disheartening areas in which to attempt
geological mapping, but it can be done ; geological structure can
be elucidated, and maps, not in great detail or of great accuracy,
but at least reliable, can be made even under such conditions.
The secret, if secret it can be called, is simply the adapting of
one's methods to the particular work that is in hand. A com-
pletely accurate map is perhaps an impossibility without great
expenditure of time and money in trace-cutting and land sur-
veying, for which the geologist may not be able to spare the
time, nor in all probability will he have the necessary instru-
ments, but a sketch map of sufficient accuracy can be pieced
together by careful, if at times laborious, work, just as a sketch
map may be made anywhere without triangulation. It is here
that the observer, who has thoroughly mastered his scale and
can map accurately on pocket compass traverses, has the advan-
tage over those who are, so to speak, tied to their instruments.
166 OIL-FINDING
If there be any road or coast- section crossing or skirting the
area to be surveyed it must be examined and mapped in detail
first, copious notes being taken of the characteristics of each
bed, such as the presence of pebbles or nodules and their
natures. In a road, even where there is no section in side
cuttings, it is possible to glean a fair amount of information.
For instance, those parts underlaid by clay can always be dis-
tinguished from parts where the underlying beds are arenaceous,
and a sharp and distinct line between thick masses of arenaceous
and argillaceous sediments can often be drawn where no actual
exposure is seen.
Then the forest or jungle must be attacked as far as
possible in the same manner as in the case of more open ground.
It is presumed that there is no topographical map available,
that no hills from the summit of which compass bearings can
be taken are to be seen, and that the courses of such streams
and rivers as flow through the area are unknown. A coast,
road, or river-section may give the key to the structure at once,
but should no such section be available, or should it be discon-
tinuous or obscure, it can only serve as a base-line on which
starting points for traverses may be selected.
To begin with, if any group of hard or massive beds be
present, the geologist should endeavour to follow it along the
strike, noting the types of vegetation it supports, the colour and
texture of the soil it forms, and whether under the weathering
processes peculiar to forest land it is capable of standing out as
a marked feature. In all thickly forested country there must
be a fairly heavy rainfall, and consequently denudation of the
surface will be fairly rapid in spite of the protection afforded
to the soil by the vegetation. An arenaceous group in these
circumstances, however soft and loosely compacted the strata
may be, will always tend to form hills and high ground as con-
trasted with argillaceous strata. Much of the rainfall is
absorbed by the porous arenaceous rocks to be thrown out as
springs at the foot of dip-slope or escarpment, whereas an
argillaceous outcrop absorbs little of the rainfall, but causes it
to flow over the surface, thus favouring sub-aerial denudation.
Consequently the outcrop of an argillaceous group among
arenaceous rocks will almost invariably be marked by a valley
or belt of low ground, however tough and hard the material
may be, and an arenaceous group among clays will stand out as
FIELD WORK 167
a ridge, however loosely compacted the strata of which it is
composed. In bare and open ground, where the rainfall is not
heavy, the relative porosities of the strata do not have such a
marked effect upon the contours of the surface.
The mapping of surface features, therefore, often becomes
very important and of the greatest help to the geologist, though
it must not be relied upon unless confirmed by other evidence
such as the nature of the soil. Where denudation is rapid it
may produce a complex system of ridges and valleys that have
little or no relation to the strike and dip of the strata ; in a
thick series of clays in which the physical characters of different
bands differ very slightly, an irregular and complicated drainage
system quite irrespective of geological structure may be
established, and the contours of the country where they can be
observed, e.g. in areas planted with sugar-cane, may be sufficient
to show that the strata are argillaceous before the soil has even
been examined.
The angle of dip has also to be considered when features
are being mapped ; the greater the angle, the more clearly
marked will be strike features, and where the strata are
practically horizontal, outcrops naturally become very irregular
and the following of them in undulating forest land may be
simply a waste of time.
Having selected a group of strata that seems likely to form
good strike features, and that is dipping at a sufficiently high
angle where it is observed in the base-line section on coast-line,
road, or river, it should be followed as far as possible along the
strike. Exposures may be few or entirely wanting, but by
studying the soil and the vegetation it may be possible to follow
a group for great distances. The occurrence here and there of
loose fragments of a hard rock, e.g. a calcareous sandstone,
along an ill-defined ridge, may enable an outcrop to be picked
up and 'mapped for miles till a river valley cutting across the
strike gives an exposure and allows an observation of dip to be
made. Once an horizon has been traced through the area to be
examined the following of other horizons becomes a much easier
task, and a fairly complete geological map may be constructed
from evidence which approached by any other method would
throw very little light upon the geological structure.
As a rule it is better not to follow the courses of streams at
first, at least not until their general directions are asceitaiced,
1 68 OIL-FINDING
If their courses be tortuous the mapping will be very tedious,
and perhaps will result in the discovery of little evidence,
while alluvial flats may be encountered to the discouragement
of the observer. Where steep dips give evidence of flexuring
on a considerable scale, however, the courses of streams or
rivers can usually be resolved into " consequent " portions,
1 i.e. across the strike, and " subsequent " portions, i.e. along the
strike ; and even where no exposures are to be seen, the evidence
from the directions of drainage taken in connection with the
orientation of ridges and hollows may give valuable evidence as
to the strike of the series.
In following up outcrops or traversing across the strike, the
geologist must map by "dead reckoning" with his pocket
compass, using his map case every fifty yards or so to mark his
track. Where the jungle is thick and has to be cutlassed to
allow passage, if two men be kept in front of the observer at
intervals of from 10 to 20 yards the mapping of track can be
simplified by omitting many of the minor turns and twists
inevitable when marching in forest land. It is not recom-
mended that traverses of more than one mile be made at first,
while three miles is as far as anyone is likely to be able to
traverse by dead reckoning with any degree of accuracy ; the
writer has found that a traverse by pocket compass of four or
five miles in forest land is inadvisable unless it is to a known
point, or to a point the position of which can be ascertained by
taking compass bearings.
The time required for simple mapping of the route taken,
without study of geological data, will vary greatly according
to the nature of the ground. Where there is not much cut-
lassing to be done and slopes are not too precipitous, one mile
an hour is a fair average pace. In difficult country and where
many observations have to be made the pace may be much
slower.
Checking a traverse can only be done by making it a
"closed traverse," coming out to some point along a road,
river, or coast-line where the position can be found, or by
making another traverse from a different starting point to the
same final point.
In any case where the geologist fails to keep his track
mapped and does not know his position, he should map on
a new sheet of paper or in a note book, and either begin a
FIELD WORK 169
fresh traverse from his unknown position to reach some point
which he can fix or recognize, or take a compass direction and
keep it as straight as he can out to road, river, or coast-line.
It is always better, however, to follow an outcrop, if one can
be recognized and followed, than to map track across bedding.
It may seem that these methods are very rough and
uncertain, and there is no doubt that the geologist when he
first undertakes work in tropical forest will make many faulty
traverses before he becomes master of the scale on which he
is working and capable of traversing forest up and down hill,
in and out of creeks and gullies while keeping his dead
reckoning with accuracy, but there is no other method that
will yield results so quickly, and at the same time develop
confidence in the observer. To map with theodolite or plane
table in the forest, cutting traces and chaining distances is far
too cumbrous and slow a method, and can only be justified
when the area to be examined is very small or when the exact
position for a test well is being determined.
In jungle work where evidence is very scanty the geologist
must be continually on the alert : nothing is too insignificant
to be noted. Every change in the colour of the soil, every
ridge that does not run parallel to the drainage channels, every
occurrence of loose pebbles or nodules should be noted and
the note marked clearly on the map. Similarly changes in
the nature of the vegetation, if they are sudden, should be
mapped. An exposed section may make clear the reason for
such a change, and a very valuable piece of evidence may be
added to the geologist's store of accumulated data. In Trinidad
the Cretaceous formation, which lies unconformably beneath
the petroliferous Tertiary Series, has frequently been recognized
by the colour of the soil and the nature of the vegetation,
when no exposures of the strata were to be seen. When
exposed the strata are often very similar to some of the
Tertiary deposits, but the soil formed by the disintegration
possesses some pecularities which distinguish it from that
formed from any of the Tertiary strata. Much of the Cretaceous
formation has been prospected for petroleum by observers who
have not learnt to distinguish it from the overlying Tertiary
rock.
In clay ground the different tints induced by weathering
processes have often proved of the greatest value, and have
i/o OIL-FINDING
enabled different bands to be mapped witb accuracy. The
black soils of a marl outcrop contrast so strikingly with the
red or yellow soils derived from a clay that there need be no
hesitation in mapping them separately. Again, " outcrops of
water," surface springs, or damp ground marked by the
occurrence of water-loving plants and trees often enable the
observer to draw a boundary line which will be found later
to coincide with the outcrop of a porous stratum.
Excavations. — The making of excavations to ascertain the
nature, dip and strike of strata is sometimes, but very rarely,
necessary. False evidence obtained by this method has often
to the writer's knowledge led observers to make very serious
and sometimes even ludicrous mistakes in their interpretation
of geological structure. It must be remembered that in forest
land, especially in tropical countries, disintegration of the
strata extends for a great distance from the surface, often
upwards of thirty feet, and in hilly ground surface-slip in
partially disintegrated rock causes an astonishing amount of
modification in the position of bedding planes. Root growth
also disturbs the strata for a considerable distance. The result
is that it is very difficult to select a spot for digging a trench
where really reliable evidence will be obtained without
excavating to a great depth. Small pits and trenches are
liable to be dug into displaced or disintegrated beds, and it
will readily be understood what confusion may arise through
accepting the false evidence obtained by this method. It is
only natural that the observer, having been at the expense
and trouble of having a few excavations made, should attach
more importance to the evidence obtained from them than to
the possibly more obscure, but certainly more reliable, evidence
that he has obtained by mapping outcrops or by the examination
of natural exposures. And thus he may acquire an entirely
incorrect idea of the geological structure.
There is something to be said for the digging of a few
pits or trenches when it is done in connexion with the
mapping of outcrops, but to depend on excavation alone to
obtain evidence is to court disaster. In mapping some 500
square miles in the island of Trinidad the author only made
use of specially dug trenches some half dozen times, and then
it was to settle some detail rather than for general purposes
of mapping. Some cuttings on roads in that Colony are
FIELD WORK
171
sufficient to prove what startling changes in strike and dip,
and even inversions, in the soft Tertiary strata are due to
surface slip.
If it becomes necessary to make an excavation, it is impor-
tant to select a spot where evidence should be obtained without
digging deep, and where such evidence is likely to prove
reliable. The bottoms of valleys are naturally to be avoided,
and also the tops of hills, hillocks, or plateaux ; in the first
case there will probably be a great accumulation of surface
wash (Fig. 15), while on the tops of hills there may be a great
FIG. 15. — Excavation for dip evidence. 1. Disintegrated strata or
surface wash.
thickness of completely disintegrated rock. At the top of a
sharp ridge or hillock, or just beneath its summit (Fig. 16),
FIG. 16. — Surface curvature, giving false dips at top of ridge.
surface curvature may vitiate the accuracy of the observation
although the strata may be obviously in situ, and at the bottom
of such a ridge water may collect so rapidly as to hinder the
digging. Half-way down a steep slope, especially if the slope
is at a high angle to the probable line of strike, gives the best
i/2 OIL-FINDING
chance of a reliable exposure, while the work of making the
excavation will be easier, and the trench or pit can be kept
drained and the exposed rock allowed to weather if the bedding
is not apparent at once. In many varieties of Tertiary strata
it is easier to detect the bedding planes after a certain amount
of weathering has taken place, so that the keeping of an exca-
vation free from water is a distinct advantage. But even with
such a favourable spot selected, false evidence may be obtained
if the strata be largely argillaceous. It is among the alterna-
tions of arenaceous and argillaceous beds, and where bands of
hard rock or nodular concretionary bands are present that the
best results are obtained from excavations.
Where flexuring has been intense, small minor folds or
wrinkles may be occasionally present in a monocline far from
any important anticlinal bend. This may lead to an incorrect
reading of the geological structure if the observer relies upon
excavations for his evidence. Fig. 17 shows a case that has
FIG. 17. — Obscure ground local flexure giving a false idea of general
structure.
actually come under the writer's observation. The ground was
low lying and evidence was very scanty ; only at the points
A and B could evidence of dip be obtained. The minor pucker
disclosed by making an excavation was taken as being the crest
of a great anticline, and as the strata on both sides were
entirely argillaceous, and gave no evidence at all that could be
considered reliable, the error survived for a long time, till field
work in neighbouring districts proved the structure to be
entirely different. But for this one unfortunate excavation no
mistake would have been made.
In all field work in forest ground, as soon as the general
structure has been ascertained, the more detailed mapping of
stream sections should be undertaken with a view to getting as
accurate an estimate as possible of the thickness of strata
exposed, and making sure of the horizons of any oil-bearing
strata that have been discovered. Should an anticlinal or dome
structure with gently-dipping flanks be indicated, the following
FIELD WORK 173
of outcrops well down the flanks should be attempted before
the inner and central portion is attacked. By this means
faults will be more easily recognized and the structure will be
more clearly and certainly delineated, and with less chance of
error than if the lower zones exposed nearer to the crest are
examined first. Where a very sharp flexure is indicated it will
be best to follow and map the crest first, as by this means any
pitches of the flexure that may be present should be detected
and the relation of surface indications to the crest will be made
clear. Afterwards, prominent beds on either flank can be
selected and their outcrops traced, and if possible correlated on
the two sides. Where anticlines are sharp, it is obvious that
the position of the crest is the most important matter, and its
trend must be mapped as carefully as possible, while where dips
are gentle and flexures broad and comparatively speaking flat,
the general form, asymmetry or pitches are of much more
importance than any mapping of a crestal line, which, at the
best, can only be marked approximately.
Lateral variations, which may be the cause of considerable
difficulty in bare and open ground, become much more serious
troubles to the geologist in heavily-wooded country, but if the
area be examined systematically, a general idea at least of such
variations should be obtained. Correlations cannot always be
established with certainty, and the field-student must not
expect to be able to correlate the two sides of any anticline in
detail. The subdivision of the series into groups may even be
impossible in some cases, except locally, but the attempt to
subdivide should always be made ; the construction of a new
road through the forest may eventually furnish excellent
evidence in side cuttings, and may enable a correlation that
has been commenced to be carried to completion and settled
beyond doubt. The mapping of any bed locally, even if it
cannot be carried far, is always advisable, but the extension of
dotted lines, indicating uncertainty as to an outcrop, between
the points where the mapping of outcrops has terminated, when
it has not been proved that the outcrops represent the same
horizon, is to be deprecated.
Generalizations on insufficient evidence are above all things
to be avoided ; it is better to leave points with regard to cor-
relation unsettled, and to say so definitely when reporting, than
to force evidence to support a conclusion, however brilliant,
1/4 OIL-FINDING
which is not absolutely certain. In cases where there is some
doubt as to the meaning of such evidence as has been collected,
a doubt that leaves the geological structure a matter of un-
certainty, a process of elimination should be employed, and
every structure possible in the particular circumstances tried
and tested both by map and section. It will always be possible
to reduce possible explanations to two or three, and the ground
must not be quitted till sufficient evidence has been obtained
to enable the geologist to decide as to which explanation is
the true one. From the map, whether completed or only half
finished, the various possible explanations can be deduced,
but it may be necessary to return again and again to certain
parts of the area to settle points which will tilt the balance
towards one or other of two alternative readings of the
geological structure. No mistake in structure is allowable,
and none should be possible if reliable methods be employed
in the survey.
Ratio of Boundary to Area. — In all mapping, whatever be
the nature of the ground, it is from the number of miles of
geological lines drawn that we get the clearest idea of the
efficiency of the geological survey. The area of land surveyed
in a given time is no test of the ability of the geologist, but the
ratio of linear miles of geological boundary lines drawn to the
square mile of area mapped shows at a glance whether evidence
has been scanty or not, and is the most certain criterion of the
care with which the mapping has been done. This ratio may
vary from fifty or sixty miles of boundary to one square mile
of area, in very complicated and well-exposed country, to
perhaps two to one in obscure and wooded ground. In the
simple geological work of a Tertiary oilfield the ratio will
seldom rise above 7 to 1. From 400 to 500 miles of geological
lines represents a good year's work for any geologist, allowing
time for the necessary indoor work, and it will be found that
this will hold good in any country and in any kind of ground,
bare or forest-grown.
To sum up, in ground thickly-clothed with vegetation the
geologist must often be content with a map by no means
complete or accurate. Mistakes in accuracy will doubtless be
made in the mapping and need never be worried over, so long
as no error is made with regard to structure. Should active
development work follow the geological prospecting of an area,
FIELD WORK 175
details of mapping can always be corrected as the ground is
opened up and new sections on roads and in excavations on
sites for tanks and buildings are laid bare. The map can
always be added to and improved in details, but if the structure
be incorrectly delineated, the responsibility for the opening up
of a field expensive to work and incapable of yielding results of
commercial importance may lie at the door of the geologist.
Thus, it is not till there is no doubt whatever about the
geological structure that the geologist has any right to speak
favourably or unfavourably of any new field : by advocating
development work without knowing what is to be tested by
the drill, or why, the geologist will class himself with the
wild-cat drillers of a former generation or the company-pro-
moting experts from whom the commercial world and the
unfortunate public have suffered only too severely and too long.
It is naturally in thickly-wooded country, where at the best
little can be known till development work has begun, that the
greatest probability of ill-advised speculation is afforded, and
consequently the more obscure the geological structure and
features, the more cautious the geologist must be in making up
his mind on the problems before him, and the more certain
must he be of the main facts before he dare venture upon
writing a report.
To visit a few oil-shows, to dig a few pits in search of
evidence, and to make a few observations of strike and dip may
suffice for some experts, but no one whose ambition is to take
rank as a geologist can afford to advise a commercial company
upon the results of what are merely preliminary observations.
If the area be tested and failure attend the attempts to strike
oil, to shelter oneself behind the alleged capricious nature of
that liquid is merely to call attention to the uncertainty of
one's own field work, and the unreliability of one's own mental
processes.
CHAPTER X
(Fen BEGINNERS)
INDOOR WOEK
THOUGH it is in the field that the real work of the geologist is
done, systematic and careful indoor work must follow if the
full fruits of his toil are to be garnered. In the last chapter
the author has endeavoured to explain the methods that he has
found most effective in field-work under different conditions; it
remains to indicate the lines upon which the necessary indoor
work can be conducted with the greatest facility, in the hope
that the field student may find in them some hints that will
prove useful to him in the more irksome but no less important
portion of his task.
When the field work in any district has been completed
there must be a gathering together and correlation of facts, a
reviewing of evidence, and a preparation of finished maps and
sections, all of which can be done much more effectively in
some office or headquarters, where there are greater facilities
and better appliances for indoor work than the geologist will
be able to carry with him in the field, however elaborate his
equipment.
As a rule it will be found that two months of actual field
work, during which an area of from twenty to fifty square
miles, according to the nature of the ground, should have been
mapped, will necessitate three weeks of indoor work. The
author has found that this proportion of indoor work to field
work holds good both in bare ground where twenty or thirty
linear miles of geological lines are mapped in a square mile of
area, and in obscure or densely forested land where the ratio of
boundary to area is 2 or 3 to 1.
Preparation of Map. — The first thing to be done is to
prepare the finished map of the area on the scale upon which
176
INDOOR WORK 177
the field work has been undertaken. This, if there are many
corrections to be made for errors in traverses by dead reckoning,
will be a matter requiring considerable care, and it may be
necessary in order to fit the traverses together with accuracy
to make a rough copy of the map first. If the area proves to
be of little importance, or if the evidence collected is insuffi-
cient to make a large-scale map, a reduction to the one-inch
scale may be expedient. In all preliminary work a map on
the scale of one inch to the mile is generally quite sufficient
to give a clear idea of the structure and the prospects of
obtaining a production of oil. Again, where a large area has
been prospected on the one-inch scale in search of localities
worthy of more careful examination, the smaller scale is quite
sufficient. Bat if the area is to be exploited and active
development work is to follow the geological examination, a
large-scale map is necessary, even though it may not be possible
to put much evidence upon it as the result of the first geological
survey.
In the finished map it may be necessary to omit much
detail that has occupied a considerable time in mapping. To
introduce detailed work where it is not essential will have the
effect of confusing those who, having little technical knowledge
of geology, may yet have to study the map and master its
significance. The map should be as simple and clear as
possible. The strata should be grouped and coloured dis-
tinctively, so that every essential point in the geological
structure is brought out. '•' Colour without line " is not allow-
able ; that is to say, every group distinguished by a separate
colour must have a clearly defined boundary up to which the
colour is brought. Mapped lines of outcrops without special
colour may be introduced locally in the midst of any group
if any object is to be gained thereby, such as showing sudden
changes of dip or explaining the broadening or narrowing of
outcrops owing to the contours of the surface. Similarly it
may be expedient to map a fossiliferous horizon in some group,
without colouring it specially. Where dips are gentle the
groups of strata coloured must be comparatively thin, but in
an area where the rocks are highly inclined it is not necessary
to colour specially more than three or four groups, and they
may be of considerable thickness.
Dip arrows and the conventional geological symbols should
N
i;8 OIL-FINDING
not be distributed too thickly about the map. There must be
enough to make the structure clear to any one without an
intimate acquaintance with geological work, and any line of
section to which special reference is to be made should have
a large number of dips noted, but the map must not be over-
loaded with such symbols. It will be found advisable to use
some characteristic and prominent symbol for surface indica -
tions of petroleum, and if the map be on a sufficiently large
scale the words "gas," "oil-seepage," "asphalt," "inanjak," or
"ozokerite," as the case may be, can be written or printed
beside the symbol. The author has always used a diagonal
cross to mark surface indications, making it rather larger and
more prominent than the symbols indicating the inclination of
strata. A symbol indicating the direction of the pitch of a
flexure is often useful.
Every map should be accompanied by a tablet showing the
groups of strata with their distinctive colours and their order of
deposition, and all symbols used.
True north should be shown on every map, but it is not
necessary to indicate magnetic north.
Sections. — When the map has been completed it is often
useful and sometimes essential to make one or more horizontal
sections through the area. These cannot be made till the map
is finished. They are very valuable as giving an idea of the
structure to those who are not capable of reading a geological
map, though they are not necessary to the experienced
geologist.
It is a common mistake of the amateur or the untrained
geologist to draw sections through a property or concession
without making a geological map at all. Such sections, though
interesting as giving evidence of the ideas of their authors as to
the geological structure of the area, are generally useless, and
are almost invariably misleading. Till the area has been care-
fully mapped the drawing of horizontal sections with any
measure of accuracy is practically impossible.
Horizontal sections should always be drawn on the same
scale as the map, and except in very rare instances for special
purposes the vertical and horizontal scales should be the same ;
for it is obviously impossible to give the true dip and thickness
of strata, or the true hade of the axis of a fold, if the vertical
scale be different from the horizontal.
INDOOR WORK
179
In making a horizontal section the contour of the surface
must first be sketched from aneroid readings, topographical
surveys, or any other evidence that is available. If there are
no ascertained data to go upon, the surface must be sketched
by guess-work. Except in very hilly ground errors will have
very little effect, as the depths beneath the surface that will
have to be considered will probably be very much greater than
the irregularities of the surface, and will make the latter appear
quite insignificant.
A base line is then drawn at a sufficient distance below the
line representing the surface ; there is no reason why this base
line should be made to coincide with sea-level or any height
above or depth below it. Then from the line of section as
A 45°
Bso°
C65'
D25° £20°
FIG. ISA.— Wrong method in section drawing.
FIG. 18B.— Right method.
drawn on the map the positions of geological boundary lines
and dips of strata as noted are marked on the base line and
projected to meet the line representing the surface of the ground.
The angles of dip are then drawn upwards from the surface
and not downwards (Fig. 18). The reason for this is that at the
surface where the dips are noted the angles of inclination are
only observed for an infinitesimal distance. The first thing
that one learns in drawing horizontal sections to scale is that
all inclined strata are parts of great curves, and that the dip of
no bed continues for any considerable distance downwards with-
out changing. The lines representing the bedding planes are
then continued downwards, care being taken to keep the thick-
ness of each group constant, unless variations in thickness have
been actually proved to exist. It will be found at once that
i8o OIL-FINDING
dips as observed are almost invariably too high to make the
drawing of a section an easy matter, and that if there be no
faulting and dislocation of the strata the minimum observed
dips will have to be accepted. This is to some extent a con-
cession to convention, but, that notwithstanding, it throws a
striking light upon the errors into which one may fall by a
blind acceptance of the dips observed at the surface as being
constant for over large distances downwards, and the danger of
depending on a few observations of dip for the elucidation of
structure. It is obvious that when it comes to the measuring
of the thicknesses of groups of strata and calculating the depth
to oil-bearing horizons throughout a field, errors made by noting
maximum dip may be sufficient to detract in no small measure
from the practical value of one's work.
There are naturally many details in a section which must
be almost purely imaginary, and such points as the under-
ground courses of unconformable junctions and fault planes
cannot be indicated with certainty unless there is direct
evidence from boring journals to assist the geologist.
The horizontal sections, if carefully constructed, will be of
great value in checking the thicknesses of groups as obtained
by measurement on the map, but the use of a section is rather
to explain the structure to those who have difficulty in reading
geological maps than to give data for the precise development
work in an oilfield. When evidence from a number of oilwells
is available, sections can be made on a much larger scale than
that used in mapping, and every petroliferous horizon can be
shown at its true depth ; the field-manager will then be able to
adapt his methods to the end in view in each well, knowing
exactly at what depths water must be shut off and where oil is
likely to be struck. In new untested fields such accuracy is
unfortunately very seldom possible.
Vertical Section. — After the horizontal section has been
completed it is often expedient to construct a vertical section
of all the strata exposed in the area, leaving room below for the
strata to be proved in the drilling. The vertical section must
be drawn to scale, but a much larger scale should be employed
than that on which the ground has been mapped. The groups
of strata, coloured as on the map and in their relative thick-
nesses, will be marked clearly in the vertical section, and the
horizons of all fossiliferous beds and oil-bearing bands observed
INDOOR WORK 181
will be noted as accurately as possible. It is advisable also to
mark the initial horizons of any wells that have been drilled,
or that it is proposed to drill, so that it will at once be apparent
what horizons have been or can be tested.
Palaeontological Work. — Any fossil evidence that has been
collected must then be gone over and compared with previous
collections or books of reference in order that any organisms
of importance in establishing stratigraphical horizons may be
recognized. Where palaeontological evidence is abundant, as
for instance in Burma, some such method as that described in
Chapter VII. may be made use of, but as a rule a less elaborate
system will be quite adequate ; it is seldom that fossil evidence
becomes of any great importance till a great mass of material
has been collected.
Petrographical Work. — It is but rarely that petrographical
work is of much value in an oilfield till after it has been at
least partially developed, but there are often points that can
be settled by the use of a microscope, and that may eventually
prove of vital importance. The examination of oilsands may
furnish very valuable evidence, as it is often possible to identify
different sands by their mineral contents. This is especially
important when an unconformability is suspected but has not
been proved. Sands that appear very similar may be from
formations of different ages, and may contain minerals which
enable them to be distinguished at once ; the heavy minerals
are frequently the most useful in this respect. The presence
of Kaolin or decomposed felspar may be a point of great im-
portance, as in some parts of Burma in separating post from
pre-volcanic strata.
The determination of the extent to which a limestone has
been dolomitized is another question that may be of vital impor-
ance in oilfields where the petroliferous rocks are calcareous. All
these matters can be dealt with by means of a petrographical
microscope without the necessity of making any chemical tests,
and though that instrument can hardly be regarded as an
essential part of the petroleum-geologist's equipment, it may
be of very great use when other evidence fails and only petro-
logical work can be depended on to solve some difficult problem.
There is, in fact, no department of geological work that cannot
182 OIL-FINDING
in certain circumstances be brought to the aid of the geologist
who is engaged in the study of oilfields.
Report Writing. — With the completion of any palaeonto-
logical or petrographical work that may have had to be under-
taken, the geologist's task is practically over for the time being ;
it only remains to write a report upon the area examined. It
is in the field work and the preparation of map and sections
that the real work of the geologist has been accomplished, but
by a very natural irony it is the report that will receive the
most attention, and the young geologist may be assured that
for one man who will study his maps, ten at least will read his
reports and interpret them in their own fashion. Chairmen of
Companies, Managing Directors, Technical Experts, Company
Promoters, and even a small section of the shareholders and
the speculative general public all attach value to a report rather
than a geological map, and consequently it is essential that
great care should be taken in the writing of it. As with most
practical geologists, among whom the writer has no further
ambition than to be classed, the hammer is mightier than the
pen, the writing of the necessary reports may be not only
difficult but irksome.
In the report on a new area, a presumed but untested oil-
field, brevity is the first essential. The geologist, if he has
sufficient time, should write out his report three times, each
time making it shorter by cutting out all that does not seem
absolutely necessary. Looked at from this point of view it is
wonderful how much "padding" can be detected in even a
workman-like and concise report.
Clearness is no less essential. Technical geological terms
should be eschewed as far as possible, as it is probable that of
those who read a report few will have more than a smattering
of geological knowledge. It is not difficult to explain in simple
language all that can be conveyed by sesquipedalian scientific
phraseology. Again, it is not enough that the writer is clear in
his own mind upon a point ; he must set it down so that the
reader cannot fail to be clear in his mind as to what is meant
to be conveyed. This is not such a simple matter as it appears
at first sight. In correspondence with reference to a report or
the ground with which it deals, the geologist's statements will be
paraphrased and unintentionally misquoted, and some day a
statement which the writer considered impossible to misconstrue
INDOOR WORK 183
will come back to him distorted out of all recognition and
labelled as his opinion. Therefore short, crisp sentences, with-
out conditional clauses, should be the rule.
Graces of style and the neat turning of phrases are to be
avoided; it is possible to give a literary flavour to scientific
work, as many of the greatest geologists, from Hugh Miller
onwards, have taught us, but it is not literature that is required
from the field geologist, but facts. If in reading over the
draft of a report one comes upon any sentence with which one
is particularly pleased, the wisest course is to cut it out at once.
Be literal rather than literary.
The point most essential of all is to stick to facts. Opinions
must not be given on any points of importance in the geology
of the area examined. It is, of course, impossible to avoid
giving an opinion upon such a question as whether an area is
sufficiently promising to warrant development work being under-
taken or not, but in dealing with questions of structure, lateral
variation, thickness of oil-bearing strata, depth to be drilled, etc.,
no mere opinion will suffice. If the certified facts cannot be
given, the geologist must say so clearly. " To the best of my
belief," "as far as I could ascertain," "in my opinion," "it
seems to me," and the numberless similar phrases should be
tabooed. Indeed, the geologist will do well to shun the use of
the first personal pronoun as much as possible, and to write his
report in the third person. The report will read better and
will appeal more forcibly to both scientific and commercial
readers if the writer does not intrude his personality, but allows
the facts as ascertained by him and set forth in map, section
and report to speak for themselves.
The ideal report must be partly descriptive ; it must explain
the map to those who may not be able to read geological maps.
It must call attention to the points of greatest importance in
the structure, etc., but it is quite unnecessary to describe and
explain the map in detail. Geological structure can be dealt
with very briefly: the map and sections should be sufficient
with a few sentences of explanation. Enough must be written
concerning the methods of mapping employed and the nature
of the strata examined to show the care with which the survey
has been conducted. The distinguishing characteristics of
different groups of strata mapped must be mentioned, but long
lithological descriptions are unnecessary.
184 OIL-FINDING
Evidence of the presence of petroleum should be treated
separately and at greater length, for much, and in some cases
perhaps undue, importance will be attached to such evidence
by those for whom the report is written. It is always necessary
to prove as conclusively as possible the petroliferous nature of
the series that has been studied geologically, and the conditions
under which .surface shows of petroleum occur afford very
valuable hints to the expert or technical adviser and the field
manager.
A comparison of the field with other areas as regards
structure, stratigraphy, and surface indications, especially if
those other areas are producing fields, may be introduced with
advantage in this section of the report in order to give some
idea of the significance of the evidence, but any canvassing
of the probabilities of proving a valuable field is better kept
for the final section.
It is always best to divide a report into clearly defined
sections, and to keep each piece of evidence rigidly to its own
section. These sections may again be subdivided, and the
report should be headed by a page showing the divisions and
subdivisions, so that any part can be referred to with the least
trouble and delay, A convenient form, which the writer has
found to meet most cases of new and untested fields, is as
follows :—
Report on Concession.
I. Introductory.
II. Formations and Strata.
III. Geological Structure.
IV. Oilshows.
V. General Conclusions and Recommendations.
Fig. 1. Map of Concession and surroundings 6 in. to the mile.
Fig. 2. Horizontal Section . » . . do.
Fig. 3. Vertical Section , . -V . 2 in. to 100 feet.
In the first section the position of the property or concession
is briefly described, and the nature of the ground, whether low
or hilly, forested or bare. The methods of survey employed
are explained and the work of any previous observers in the
same area must be touched on.
In the second section the various formations exposed in the
INDOOR WORK 185
area are described shortly in their stratigraphical relations.
Each group of strata mapped and coloured separately is described
and its thickness estimated, and the horizons of oil-bearing
strata and fossiliferous beds are given. If fossil evidence be
very abundant, it is better not to give it at length in this
section, but to state the general conclusions arrived at from
palaeontological work, and keep a detailed account of it for an
appendix to the report.
In Section III the structure as shown by the map and
horizontal section is explained briefly, and the account may be
subdivided into evidence of: (1) Flexuring; (2) Faulting, and
(3) Unconformabilities, etc. as may be necessary.
A special section upon the indications of petroleum is only
necessary when they are extensive and important enough to
deserve careful description. If the "oilshows" are few and
insignificant this section can be merged in Section II. '
In the last section the general conclusions on scientific
points must be stated very clearly and briefly: it is often
advisable to number them, e.g. : —
(1) The strata are of the nature common to many oilfields,
and give evidence of containing petroleum at intervals
throughout a thickness of 3000 feet.
(2) The geological structure over the greater part of the
area is unfavourable to a production of petroleum, but
in the north-west corner of the concession is very
favourable.
(3) The area of favourable structure is approximately
acres, etc., etc.
Though the scientific reader will doubtless give full
attention to the whole report, it is the last section, the " con-
clusions and recommendations" that will be studied most
closely, and that will be quoted and canvassed by every one
else; indeed, the earlier part of the report may merely be
glanced through.
After the " conclusions " comes the " opinion " as to whether
development work on the new field will be justified or not. If
properly led up to, this opinion should appear inevitable.
Then, if a favourable opinion has been given, come the
recommendations as to how the area should be developed. The
sites chosen for test-wells should be described, and the reasons
1 86 OIL-FINDING
for selecting them given. If locations have actually been
marked on the ground and on the map, it is not necessary to
deal at length with their advantages and disadvantages; the
initial horizon of each test-well can be shown on the vertical
section, and the position of each as regards geological structure
can be given on the horizontal section.
The depth to be drilled in each case should be stated, as
well as the nature of the strata to be drilled through, and any
difficulties likely to be encountered in the drilling through
the occurrence of water-sands, loosely-compacted sands, thick
soft clays, or steeply-dipping strata must be mentioned.
Proximity to water supply, best means of access to the well
sites, and difficulties in the way of road-making incidental to
the nature of the country and strata should be touched upon :
though these matters are hardly within the province of the
geologist, any information about them will be of value to a field
manager.
Finally, if the geologist has sufficient experience in oilfield
work to justify him in so doing, the method of drilling which
he believes will give the most successful results in the special
circumstances, and the expenditure which he considers should
be sufficient to allow of the test-wells being drilled in a
satisfactory manner, may be indicated. On these latter points,
however, it is well to use a wise caution. Unforeseen circum-
stances may arise to falsify estimates of expenditure, and it is
better, unless specially requested to do otherwise, to leave all
such matters to those who will have to be responsible for the
practical development work.
It is a very simple matter when dealing with a new oilfield
to write a favourable report in somewhat indefinite terms,
dealing with such evidence as has been obtained in a general
way, and not stating the reasons why any particular fact is
regarded as favourable. Eeports of this kind are very common
nowadays, and may frequently be found in a prospectus.
The geologist who wishes to establish his reputation for
reliability must be careful not to fall into this style, which is
fatally easy to acquire. The disadvantages of a new field should
be stated as clearly as its advantages, and though the expert
who does not hesitate to condemn a field upon evidence which
he gives, and holds to be sufficient, is never so popular as he
who can write a carefully safe-guarded report, which at the
INDOOR WORK 187
same time gives the reader a highly favourable impression of
the prospects of a field, in the long run the man who confines
himself to the stating of facts, and has the courage of his
convictions, will carry the most weight. A reputation for
caution and even pessimism will be of more value to the young
geologist than an ill-regulated enthusiasm which may have the
effect of inducing capitalists and the public to sink large sums
in fruitless expenditure.
Report on a proved field. — In reporting upon a field already
tested and partially developed, the geologist has a much more
complicated task. An accurate topographical map in all
probability will be available, and the geological data must
be noted upon it with great care. A larger scale than 6 or
8 inches to the mile will probably have to be employed, and
the exact position of every well, drilled or drilling, must be
marked. Then after the geological map and horizontal section
have been completed, logs and boring journals must be con-
sulted and every well projected on to the horizontal section,
showing its initial horizon and the depth reached. The
underground geology can then be added from the logs of the
wells and a correlation of the oil-bearing horizons attempted.
Where many wells have been drilled it is often possible to
correlate every water-sand and every oil or gas-show throughout
a field, and to draw contour lines upon the map showing the
depths to an oil horizon at any part of the area. In some of
the American fields, notably that Coalinga in California, this
has been done with great success. When such accurate work
is possible the required depth for each new well can be
calculated from the elevation of its site and its position with
regard to these contour lines, and the depth at which water
must be shut off can be given with certainty, so that a field
manager is enabled to save much expense by adapting his
methods to the particular object aimed at and economizing in
the matter of casing.
The report will require to be written on a different system ;
there must be a section dealing at length with the evidence
from wells and the correlation of the underground strata.
These, however, are matters not entirely geological, and can
be undertaken by persons without any special technical know-
ledge. It is before a field has reached the producing stage
that the services of a geologist are essential. After the map
i88 OIL-FINDING
and horizontal sections have been completed, and the confines
of the field proved, the petroleum expert may take the place
of the geologist.
When working in a producing field great caution must be
exercised in taking hearsay evidence about the strata in any
well and the shows of water at any horizon in it. The logs
of wells are not always reliable, and even when kept with
care too much is often left to the personal opinion of the
driller. Strata are frequently incorrectly described, and two
drillers may give different names to the same type of sediment.
Boring records and hearsay evidence, therefore, must not be
blindly relied upon. Not that the geologist will be intentionally
misled by the practical workers in an oilfield — though cases
of deliberate attempts to mislead the scientific worker are not
altogether unknown — but the mind untrained in scientific work
may not be able to convey or express information in such a
form that it can be grasped accurately. From a report hear-
say evidence should be rigidly excluded ; it is better to leave
a point unsettled than to rely, however slightly, upon second-
hand information.
One point with regard to the writing of reports remains
to be touched upon. It will frequently happen that the
geologist in the course of his field work will establish, or obtain
evidence about, some point of general scientific interest, and
he will naturally be tempted to enlarge upon it in his report.
In such cases the best procedure is to consider whether the
scientific point in question is of practical importance in the
commercial development of any particular field, and whether
other members of a scientific staff working in the same interests
will be helped in their investigations by the new knowledge
acquired. If so, the evidence should be described briefly and
the conclusion stated. Otherwise it is better not to overload
a report with matters, however interesting and important from
the scientific point of view, that have no direct bearing upon
the practical finding and producing of petroleum. Appendices
can always be written to a report to contain such results of
the geologist's investigations as are of greater scientific than
practical importance.
Reports are always subject to criticism, and as a matter of
course always receive it, practical or academic, pertinent or
impertinent, fair or unfair, and occasionally merely ignorant.
INDOOR WORK 189
Any criticism is stimulating, or should be so, to the practical
geologist, and in the majority of cases must be beneficial how-
soever unfair it may be. The answer to it is in work rather
than controversy. Theories may be promulgated, tested by the
facts, and fall ; fallacies often die very hard and may even be
brought to life again unexpectedly, but the search for truth
goes on, and the dealer in facts has in the end the victory over
the critic steeped in theory who has not the advantage of first-
hand acquaintance with all the evidence. Therefore the field-
student in his writings should eschew theory and stick to facts,
nor resent the spur of criticism however clumsily applied.
In these notes the author is conscious that he is setting
forth, probably at undue length, a great deal of very obvious
advice, which even the tyro in geological work in oilfields may
stigmatize as common-place and banal. " These matters," he
may say, "are merely common sense," in which he neither
requires nor desires instruction. The author does not cavil at,
but rather applauds such a dictum ; each man must depend on
his own common sense, and to teach geology from books rather
than in the field is an academic absurdity. Out of the fruits
of considerable experience the author has written in the last
two chapters these notes, not claiming for them any originality,
nor desiring to dogmatize, but hoping that here and there
among them the beginner may find something that will help
him in his practical work.
It may seem that the duties of a petroleum geologist have
been made to appear samewhat elaborate and complicated.
They may be, and indeed often are so ; the geological work in
an oilfield, especially in a Tertiary oilfield, is in itself simple,
but to guide the development work of a petroleum company
with complete success, without causing needless expenditure,
and without having to admit failure now and then, may be very
difficult. Every kind of evidence must be studied, every
precaution taken, and every detail examined if certainty is to
be arrived at. And that in very many instances practical
certainty can be attained in oil-finding is the firm belief of the
author, though years of laborious fieldwork and research under
conditions not always of the most attractive may have to be
accomplished before such a result is within sight.
A great field is opening up nowadays for the prospecting
geologist, the man trained in scientific processes of thought,
OIL-FINDING
and physically fitted to endure the hardships and discomforts
of field work in those parts of the world where nature is not yet
shackled by civilization. It is in tropical and sub-tropical
countries that much of the earth's richest stores are to be
searched for and won, and it is to him who can withstand
unfavourable climatic conditions, under tropical sun, or in dark
forest, on desert and barren hill, or in cane-field and plantation,
that the prizes will- fall.
In no branch of geological work is there a more promising
field than that offered by the search for petroleum, and no com-
mercial enterprise depends more for its success upon the
geologist than the finding and winning of oil. The life of the
oil-finder, with its travel in many lands, its contact with
many races, and its frequent change of scene, is, taking the
rough with the smooth, a thoroughly enjoyable one. To
the sportsman — and every field-geologist should be somewhat
of a sportsman at heart — there are moments that compensate
one richly for the hardships incidental to the exploration
of wild and little known country.
If this little introduction to the great subject of oil-finding
be instrumental in turning the attention of the young geologist
to the fascinating subject of petroleum, and be of service, in
however slight a degree, in setting his feet in the path that
leads to success, the aim of the author will be accomplished
and his labour rewarded.
INDEX
ABNEY'S level, 154
Adsorptive properties of clay, etc., for
bitumen, 21, 29, 41, 97
Alaska, 41
Alluvium, 165
Alteration in character of sediment,
124
Ammonia as evidence of animal
matter, 21
Ammonium sulphate from oil-shales
and from peat, 27
Angle of dip, changes in, 75, 142, 144
Anglo-Persian Oil Company, 47
Animal matter, theories of origin
from, 4 ; present in oil-shales, 21
Anthracites and bituminous coal,
origin of difference, 20
Anticlines, 70 ; asymmetrical, 72, 135,
137 ; compound, 73 ; symmetri-
cal, 71, 134, 140
Arakan Yomas, 16, 63, 83
Arenaceous beds, deposition of, 50
cover, 19, 29
Argiline, 43
Argillaceous cover, 19, 29, 30
matter and formation of petro-
leum, 29
rock as a filter, 42
Arkoses, 118
Asphalt deposits, 89, 93-103
Asphaltene, 112
Asphalt from Pitch Lake, Trinidad,
51 ; analysis of, 95, 96
Asphaltic oils, filtration by clay, 29,
41 ; as contrasted with paraffin
oils, 36, 53, 91, 115, 149; from
monoclines, 74
Asmari limestone, 48, 84
Asymmetrical anticlines, 72, 135-139
BAKU, 40, 145; oil-wells, 51; sands,
51, 52
Baluchistan, 16, 34, 41, 43, 49, 59, 69,
122, 160
Barbados, 3, 18, 108; manjak, 112;
tar-sand, 109, 113 ; unconformity,
86
Bassein Series, 83
Bitumen, adsorption of , 21 ; of Pitch
Lake, Trinidad, 51, 95, 96
Bituminous compounds, and igneous
action, 3, 25; from vegetable
matter, 12 ; from peat, 27
Bituminous outcrops and impregna-
tion, 89, 108-110
" Blackband," 12
Blue clays, 7
" Boiling Spring," Barbados, 108
Borehole indications, 115
Borneo, 20
Boundary to area ratio, 174
Brine, associated with petroleum, 32-
35, 115
Burma, 2, 15, 16, 68, 69, 74, 160, 181 ;
Bassein Series, 83 ; clay conglo-
merates, 62 ; correlation of series
by fauna, 125-131; faults, 78;
mud-volcanoes, 107 ; paraffin oil,
36, 91, 110; Prome Series, 121;
Sabe field, 85 ; unconformity, 81-
83; Tertiary Series, 39, 63;
Twingon, 149; vide Irrawaddy
Series, Pegu Series, Yaw, Yenang-
young, Yenankyat.
Burmah Oil Company, 15, 125 if, 146
Burnt Cliff, Barbados, 18
CADMAN, Professor J., viii., 18
Calcareous cement and concretions,
54, 55, 123
Californian oil-fields, 5, 39, 51, 74
102
Carbonaceous phases passing into
petroliferous, 15, 20, 31
shales, formation of, 12, 14, 15
Carboniferous measures, vide Coal
seams.
Carmody, Prof., 7, 52, 92, 104, 113
Cedros, Trinidad, 17, 19, 55, 105
Chemical researches on origin of
petroleum, 25
" Chemin de Diable," 106
Clapp, F. G., 85
Clay, 14, 54 ; Kimeridge, 17, 22
Clay conglomerate, 62
Clay-gall beds, 62
Clay-ironstone, 118
Clifton sand, 85
Clinometer, 153, 154
Coal, associated with petroleum, 20, 21
191
1 92
INDEX
Coal-seams, formation of, 12, 14;
connected with petroleum, 20
Coalinga, California, 187
Columbia Estate, 105
Columnar jointing, 112
Compass, prismatic, 153, 157
Compound anticlines, 73
Conglomerates, oil-bearing, 47
Contour of sand-grain, 51, 97
Correlation of strata, on lithological
grounds, unsatisfactory, 65, 118 ;
by fossil fauna, 120-130
" Cover," effect of, 19, 28-30; as con-
cealing petroleum, 116
Cretaceous formation, 41, 58, 66, 169
" Crevices," 44
Crouliansky, M., viii
Cunapo lignite field, 61
" DEATH-MARK," 8
Deltas and deltaic conditions, 13, 23,
59, 118 ; fossil evidence of direc-
tion of formation, 62 ; sedimenta-
tion in, 28 ; vide Pegu Series
Depth of well, calculation for, 143,
144
Depth-temperature, 26, 28, 32
Desiccation, 34
" Devil's Woodyard," 106, 114
Diatoms, 5, 8
" Die Fossilen von Java," 128
Dip, angle of, 75, 142, 144 ; estima-
tion of, 163
Distillation, local, from igneous or
volcanic action, 3, 25
Dolomites, 47
Dolomitization of limestones as affect-
ing storage, 47, 48
Dome structure, 70, 71, 80 ; location
of well on, 134-137, 140
Drill, kind of, 150
EARTH-MOVEMENTS, 28, 34, 57, 64;
their study, 67-69
Eastern States of America, oilfields
in, 38, 71, 72
Eldridge, Mr., Ill
Engler and Hofer, 1, 5, 25
Equipment for prospecting, etc., 152-
156
Estuaries, sludge from, 6, 7; free
from seaweeds, 23; in Tertiary
times, 59
Evolution of gas, vide Gas evolution
Excavations, 170
Eye training, 160
FACHER or fan structure, 68
Faule Island, 43
Fault-fissures, 45
Faults, as the geologist's deus ex
machina, 44, 70 ; as affecting lo-
cation of wells, 145 ;^as affecting
storage, 45; as part of earth-
movement, 69 ; their true nature
and effect, 76-80, 146
Fauna, as aids to stratigraphy, 124 ;
as evidence for animal origin of
petroleum, 8 ; as indicating direc-
tion of delta formation, 62 ;
example of use from Burmese
Tertiaries, 126-130
Field-mapping, vide Map-making
Filtration of oil, 41, 91, 138
Fish, as origin of petroleum, 10
" Fissures," 44
Flexures, 68, 69, 76-80 ; as affecting
well-sites, 135-140; in mapping,
172
Folds and folding, 68, 69, 76-80; in
Barbados, 86
Foraminifera, 5, 8, 39
Fossil fauna, vide Fauna
Fucoids, theory of origin from, 22-24 ;
Cambrian beds, 22
Fyzabad, Trinidad, 102
GALEOTA oil-bearing group, 91 ; oil-
sand, 104
Galfa Point, Trinidad, 55, 107
Galicia, 41, 139
Gas evolution, 89, 101, 103, 117
Gas-pressure, 38, 40, 117, 134
Gas-sands, 50
Gas-shows, 108, 117, 187
Gasteropods, 8, 62, 128
Gas wells, 107
Geological Survey of Great Britain,
viii, 20, 152, 154
Geological Survey of India, 72, 81,
126
Ghasij shales, 16
Gilsonite, 45, 110, 111
Glauconite, 59, 118
Grahamite, 110
Grande Riviere, 16
Griswold, W. T., 73
Grits, oil-bearing, 47, 118
Guapo, Bay, 96 ; Oil Company, 30
Guayaguayare, Trinidad, 108
Gypsum, 59, 118
HADE of axis, decrease and direction
of, 137, 139
" Hard shells," 15
Harnai Valley coal, 49
Hydrostatic pressure, 39, 139
Hypogene origin, theory of, 2
IGNEOUS action, as causing distilla-
tion, 3, 25
Impregation of rocks, 46, 47
Indications in a borehole, ] 15
Inorganic origin, theories of, 2
Intrusion of veins of manjak, 45
Irois, Trinidad, 18
INDEX
193
Irrawaddy, 13, 64; Series, 69, 78,
unconformable with Pegu Series,
81, 83, 125
JAMES, S. Lister, viii
Japan, 2
Java, 54
Jemsah, 48
Jungles, surveying in, 165, 168, 169
KALA DERIBID, Persia, 42, 91
Kaolin, 181
Karroo, South Africa, 3 •
Kasr-i-Cherin, Persia, 72
Khatan, 34, 43, 49
Kimeridge Clay, 17, 18, 22
Kirta, Baluchistan, 49
LA BBEA OIL-FIELDS, 18; oilsand,
51-53, 94, 97, 98, 114; pitch-
lands, 98
La Lune, Trinidad, 103, 104
Lagon Bouff , 107
Lagoons, as illustrating accumulating
vegetable matter, 13, 14
Lamellibranchs, 14, 62, 128; with
" death-mark, "8
Lateral variation, 58-66 ; evidence of,
61, 62; in deltas, 60, 61; in
Pegu Series, 125 ; importance of,
65
Lenticularity, 141, 142
Lignite, beds, 11; their formation,
14, 15; connection with petro-
liferous beds, 16-19 ; Cunapo
field, 61
Limestones, occurrence of oil in, 5,
47 ; advantage over sandstones,
55 ; as affecting quality of oil,
48 ; at Maidan-i-Naphtun, 48 ;
Marine, 118; Trenton, 47 ; Asmari,
48, 84 ; Spindle Top, 48
L'Islet Point, 107
Lithological correlations unsatis-
factory, 65, 122
Littoral deposits, evidence from, 8 ;
formation of, 13-15
Lizard Eiver and Spring, 91, 92
Location of wells, 132-149 ; on asym-
metrical anticline, 135-139 ; dis-
tance apart, 149 ; in faulted areas,
145, 146 ; on a monocline or
terrace structure, 142, 143; on
symmetrical anticline or dome,
134, 140 ; to determine extent of
field, 140, 148
Louis and Gordon, Messrs., 94, 98
Louisiana oilfields, 5, 39
Lunn, R., viii
Luristan, Persia, 33
MACRORIE, B. F. N., 146
Mague District, Burma, 78, 83
Maidan-i-Naphtun oilfield, 34, 47,
48; sharp folds in, 73; sulphur
at, 49, 108 ; surface indications
at, 90, 108 ; unconformity in, 84
Maikop, 74
Mangrove swamps, 13
Manjak, 45 ; veins of, 89, 110-114
Map-cases, 152
Map-making, in the field, 152, 155,
158-170 ; traverses, 162, 168 ;
indoor, 176 ; sections, 178
Maps, use of, 57, 151, 187; import-
ance of geological, 87, 116, 144,
151
Marbella Mine, 113
Marcasite, 49
Marmatain, Persia, 47 ; sulphur at,
49, 108
Martin, Dr., 128
Mexico, 2, 74 ; asphalt deposits, 102
Migration of oil, 38-46 ; as affecting
well-sites, 134-139
Millstone grit, 65
Minbu, Burma, 107
Mineralization, state of, 122
Miocene strata, 14, 69
Mollusca, 8
Monoclines, 74 ; locating well on, 142
Morne L'Enfer, Trinidad, 103
Mud- volcanoes, of solfataric type, 2,
103 ; due to discharge of gas, 2, 41,
103 ; analysis of water from, 104 ;
as a surface indication, 89, 103 ;
associated with salt, 32 ; at Pitch
Lake, 99; in Burma and Trini-
dad, 105-107 ; size of, 105
NESS, J., viii
NhangeUite, 22
Noetling, Dr., 126, 127, 129
OHIO, 72, 85, 108
Oilfields, near volcanic lines, 2; in
limestones, 5 ; of Baku, 14, 40,
51, 52 ; of Louisiana, 5, 39 ; of
California, 5, 39, 40, 50; of
Eastern United States, 38, 40,
71, 72; of Texas, 5, 33, vide
Spindle Top ; Rio Blanco group,
17, 52 ; La Brea group, 18, 51-53 ;
associated with lignitic strata,
16-19, with coal seams, 20, with
salt and brine, 32 ; of Persia,
vide Luristan,Maidan-i-Naphtun;
of Baluchistan, vide Khatan and
Baluchistan, vide Burma, Trini-
dad, Maikop
Oilsands, 50-53
Oilshales, ammonia in, 22, 27;
Scotch, 3, 22
Oilshows, vide Shows
Omnimeter, 154
Organic origin, theories of, 3-24
0
i94
INDEX
Origin of petroleum, theories of, 1 ;
from terrestrial vegetable matter,
11-22; from sea-weeds, 22-24;
inorganic, from hypogene causes,
2, by volcanic action, 2 ; organic,
from animal matter, 4-10, fish,
10 ; fossiliferous strata, as bearing
on, 8 ; artificial, from peat, 27
Orinoco, 13, 61
Oropuche, Trinidad, 17, 102, 107
Oyster beds, 62
Ozokerite, 45 ; as a surface indication,
110, 114
PAKOKKU DISTEICT, Burma, 83
" Palceontographica Indica," 126, 127
Palaeontology, 125, 131, 181, vide
Fauna
Pala Seco, Trinidad, 75
Paraffin oils as contrasted with
asphaltic oils, 36, 53, 91, 115, 149
— , solid, percentage of, 36, 91
Pascoe, E. H., 72, 135
Pauk, 16
Peat, processes for utilizing, 27 ; as
illustrating formation of petro-
leum, 32
Pegu Series of Burma, 8, 31, 41, 78,
121, 124: geological history of,
63 ; earth-movement in, 69 ;
dome structure, 80 ; uncon-
formity with Irrawaddy Series,
81, 83, 125 ; stratigraphy of, 126-
128
Pencils, coloured, 155, 159
Pennsylvania, 40, 72
Persian oilfields, 33, 34, 59, 122, 160 ;
clay conglomerates, 62 ; flexures,
68
Peru, 74
Petrography, 181
Petrolene, 112
Petroleum, asphaltic and paraffin
contrasted, 36, 53, 74, 91, 108;
from lime and sandstones con-
trasted, 48, 49; its origin, vide
Origin ; percentage of paraffin,
16, 36 ; process of formation,
25-36 ; quality of, as determined
by pressure, 28, as affecting
migration, 71
Petroliferous phases passing into
carbonaceous and lignitic, 15-19,
31
Phosphates, difficulty from, in animal-
origin theory, 9, 10
Piparo, Trinidad, 41, 103
Pitch-lands, at La Brea, 98
Pitch Lake of Trinidad, 51, 53, 114 ;
its formation, 93-100
Plane-tables, 153, 169
Point Ligoure, Trinidad, section at,
20, 30 ; depth temperature at,
32 ; oil in sea, 93
Poole District, Trinidad, 114
Porcellanites of Trinidad, 17-19, 29 ;
La Brea, 94
Porosity of oilrocks, as affecting
migration, 39, 71 ; as affecting
sand brought up, 52 ; as affecting
storage, 46-48
Port of Spain Harbour, 7
Portuguese South Africa, 22
Pressure, as condition of formation
of petroleum, 25, 28; amount
required, 30; gas pressure, 38,
40, 134 ; hydrostatic, 28, 38
Princestown, 106
Prome, series, 121 ; district, 126
Prospecting, 56-59 ; hints for, 151-
175
Protractor, 154
Pyrites, 49
RAMRI ISLAND, 42
Range-tables of fauna, 128-131
Redwood, Sir Boverton, v
Report writing, 182 ; on a proved
field, 187
Reservoir rocks, 47-51
Reynolds, G. B.,viii, 72
Richardson, Prof. Clifford, 29, 41, 51,
95-97
Rio Blanco, 17, 93 ; oilsands, 52, 97
Rogers, C. S., viii
Russia, 20, 74
SABE FIELD, Burma, 85
Sakhalin, 2
Salt, associated with petroleum, 32,
35, 115
San Fernando, Trinidad, 19, 43;
manjak, 111, 113; Vistabella
vein, 112
Sand grains, contour of, 51, 97
Sandstones, oil-bearing, 47, 50; as
affecting quality of oil, 48, 49 ;
porosity of, 52-55
Sangre Grande, Trinidad, 14, 18,
106
Sargasso Sea, 23
Scotch oil-shales, 3, 22, 25
Sealing up of strata by impervious
cover, 19, 25, 28-30
Seaweed origin, theory of, 22-24
Sections in map-making, 178
Sedimentary beds, formation of, 13
Seepages of oil, as a surface indica-
tion, 90-93
Selenite, 59, 118
" Shows," 40, 46, 88, 116, 117
Sind, 69
Singu oilfields, 16, 72
Siparia, 18
Sitshayan shales, 121
INDEX
195
Sludge, examination of, 6
Sp. gravity of oil, 39, 92 ; as affecting
migrat on, 71 ; as dependent on
depth, 29
Spindle Top, 149 ; dome structure,
71; limestone, 48 ; sulphur at,
49
Spintangi, Baluchistan, 49
Stratigraphy, its importance, 121 ;
evidence for it, 122-131
Strike, change of, 75, 142
Strike-lines, determination of, 57, 58,
68
Structure, geological, or secondary
importance, 67
Structures, favourable to concentra-
tion of petroleum, 70- 76; location
of well on, 134-140 ; vide Dome,
Terrace, Anticline, Monocline
Subterranean storage, 46-51
Sulphur and its compounds in petro-
leum, 24 ; as a surface indication,
109 ; in limestones, 49, 110 ; in
sands, 42, 49
Sulphuretted hydrogen, 108
Surface indications, 88-115
Surveying in open ground, 157 ; in
. jungles, 164-169, 172, 174 ; topo-
graphical, 158
Symmetrical anticlines, 71, 134, 140
Synclines, 73
TACHYOMETEE, 154
" Tar sands " of Barbados, 109, 113
Temperature, vide Depth-tempera-
ture
Terrace structure, 72, 142
Tertiary strata in Burma and Trini-
dad, 7, 13, 39, 41, 43, 48, 58, 61,
64, 86, 164, 172, 189 ; and earth-
movement, 68 ; and surveying,
169, 171; and mineralization,
124; their formation, 14, 17,
19
Texas oilfields, 5, 33, vide Spindle
Top
" The Modern Asphalt Pavement," 51,
96,97
Thompson, A. Beeby, 51, 52
Tobago, 93
" Torpedoing," a well, 53
Traverses, 162, 168
Trenton limestone, 47
Trinidad, 6, 44, 74, 75 ; as evidence
for accumulating vegetable
matter, 13 ; asphalt deposits,
102 ; filtered oil, 91 ; lignite dis-
trict, 18, 61; paraffin oils, 36,
110; sands and sandstone, 51,
54, 109; seepage of oil, 90; sur-
veying in, 169, 170; Tertiary
Series, 7, 14-17, 39, 41, 61, 64 ;
vide La Brea, Manjak, Mud-
volcanoes, Pitch Lake, Point
Ligoure, Porcellanites
Trinity Hill Forest Eeserve, 90, 107
Twingon, oilfield, 149
UINTAITE, 110
Unconformabilities, 69, 81-86 i at
Maidan-i-Naphtun, 84 ; in Ohio,
85 ; in Barbados, 86 ; in Pegu
and Irrawaddy Series, 81-83
" Underclay," 14, 123
VANCE KIVEB, 93
Vegetable origin of petroleum, 11-
22
Veins of manjak and ozokerite, 110
Venezuelan pitch lakes, 101
Vertical sections, 180
Vessiny Elver, 94
Vistabella vein, 112, 113
Volcanic action, as origin of petro-
leum, 2, 3
WALL and Sawkins, Messrs., 17, 43
Water, necessary in formation of
petroleum, 27 ; in limited
quantity, 35 ; in synclines, 73
Water-sands, 50
Well-sites, 133-150, vide Location of
Wells
West Indies, 2, vide Barbados
West Virginia, 20
Winda, Mr., 5
Wolgan Valley, Australia, 21
" Wrench-faults," 77
YAW Valley, Burma, 16, 31 ; depth-
temperature at, 32 ; sandstone of,
16, 82, 124
Yedwet inlier, 78, 80
Yenangyoung oilfields, 16, 33, 146 ;
Yenankyat oilfields, 16, 72, 85
ZONES of fauna, 126, 127
PRINTED BT WILLIAM CLOWES AND SONS, LIMITED, LONDON AND BECCLES.
0
YC
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